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Quantum Physics Just Got Less Complicated
wabrandsma sends this news from Phys.org:
Here's a nice surprise: quantum physics is less complicated than we thought. An international team of researchers has proved that two peculiar features of the quantum world previously considered distinct are different manifestations of the same thing. The result is published 19 December in Nature Communications. Patrick Coles, Jedrzej Kaniewski, and Stephanie Wehner made the breakthrough while at the Center for Quantum Technologies at the National University of Singapore. They found that wave-particle duality is simply the quantum uncertainty principle in disguise, reducing two mysteries to one.
In its current, immature state, the pilot-wave formulation of quantum mechanics only describes simple interactions between matter and electromagnetic fields, according to David Wallace, a philosopher of physics at the University of Oxford in England, and cannot even capture the physics of an ordinary light bulb. "It is not by itself capable of representing very much physics," Wallace said. "In my own view, this is the most severe problem for the theory, though, to be fair, it remains an active research area."
A little early to "drop it", it seems.
~ Whence do you come, slayer of men, or where are you going, conqueror of space?
There is a difference between having a competing theory, and proving that two broadly recognized phenomena are actually mathematically equivalent.
--- Most topics have many sides worth arguing, allow me to take one opposite you.
"wave-particle duality is simply the quantum uncertainty principle" gets a "no shit" straight away from me, though I guess a rigorous proof of it is kind of news.
That's how science work. You don't base your decision on the mere principle that it more or less looks kind of logical.
(After all, it only looks "kind of logical" to your *brain*, which has spent the last few million years being optimized to help bipedal monkey survive together in the savanah. Actual science can some time feel "weird" and defy logic, because it defies the monkey-brain logic. - e.g.: the sum of all positive integer is a negative fraction)
You do thoroughly prove that by the numbers.
Yes, the double-slit experiment (where single particle behave like waves) strongly suggest that the uncertainty principle is at work (there's not *a signle photon* going through the slits, it's instead a function showing the distribution of the probabilities to pick it up at a certain place).
Now, we have mathematical proof that's indeed the case.
Science: the only place where it's actually correct to spend the time and mental ressource to formally prove that water *is* wet, and fire *does* burn. Because, along the way, you develop mathematical tools which come handy to do more advanced science.
"Sufficiently advanced satire is indistinguishable from reality." - [Tips: 1DrYakQDKCQ6y52z6QbnkxHXAocMZJE61o ]
A quantum state of position can be written as a superposition of a momentum states; the position is certain and the momentum is uncertain.
A quantum state of momentum can be written as a superposition of position states; the momentum is certain and the position is uncertain.
That's the duality and the extremes of the uncertainty principle. The mathematics can also show more generally, that the uncertainty in position and momentum is always more that a certain value (Planck's constant).
These things follow directly from the axioms of Quantum theory, Hilbert spaces and any two non-commutative operators. So I really don't see how Quantum Physics "just got less complicated". It's the same as it's always been. Although I've not read the paper yet, maybe that makes more sense.
Who ordered that?
Ok, let me give this a crack.
You build a box. That box contains a Geiger counter, which clicks if it detects the decay of a particle. Because you're a sick, sadistic fuck, you hook up that Geiger counter to a hammer such that if the Geiger counter detects the decay, it engages the hammer to smash a vial of poison, thus releasing it into the box. You then--because, as I said, have issues with sociopathy--put a cat in the box and close the lid. The box is very thick, completely opaque and completely soundproof. You have no way of knowing what's going on inside the box.
You wait an hour. In that hour, you do some maths that shows that there was a 50% chance that the particle decayed, triggering the Geiger counter, which triggered the hammer to break the vial of poison, releasing the gas and killing the cat.
The question becomes: before you open the box, is the cat alive or dead? Or is it somehow...both?
Your gut instinct is to say, "That's stupid. Of course it's either alive or dead. How the fuck could it be both?"
But the thing is, there are certain, non-cat-related experiments that we've done that REQUIRE the answer to be BOTH. Perhaps the simplest (and certainly the one we physicists learn about first) is the double-slit experiment. The basic idea is, you shoot a beam of something (light, gold atoms, DNA, doesn't really matter) at a slit, and it forms a pattern on a wall. It'll form this pattern even if you shoot your particles one at a time. Then, you close that slit and open another one, and fire your beam again. It forms a different pattern.
Now you open BOTH slits and fire your beam. What happens? Well, what you'd expect is to get a pattern that's the SUM of the pattern you get through each slit. That corresponds to the idea that the particles each go through either Slit 1 or Slit 2. But instead what you get is an INTERFERENCE pattern, which can ONLY happen if the particles are going through BOTH HOLES. And recall I said earlier--you get the same pattern even if you shoot your particles one at a time, which means THE PARTICLE MUST BE INTERFERING WITH ITSELF.
So back to the cat: is it alive or dead, or is it alive AND dead? According to the Copenhagen Interpretation, it's both. But that's why the cat thought experiment was devised in the first place: to highlight how RIDICULOUS that was. The crazy thing is that, seventy years later, we don't really have a better interpretation (at least not one that's widely accepted). So until someone builds this possibly-cat-killing box, we won't really know if the Copenhagen Interpretation is right, or whether something even stranger goes on when quantum events get amplified to the macro level.
One final note: practically speaking, there's no way to build this experiment, because of the whole "you have no way of knowing if the cat is alive or dead without opening the box" part. Isolating a system as big as a cat-box from the rest of the universe is not really feasible. You would also have to construct a particle decay detector that did not, itself, "collapse" the waveform of the decaying particle (otherwise the paradox is resolved before you ever make it to the cat).
Hope that was helpful!
One particle doesn't interfere with itself, and can't because the interference pattern is seen in the density of collisions over an area.
As many of these single dots build up, they tend to cluster around an interference pattern - as if some particles went through one slit, and some particles went through the other slit.
Not quite--and that's really the key element of this whole thing: the particle somehow DOES interfere with itself, because the interference pattern that builds up, just one particle / one dot at a time is DIFFERENT than what you'd get if each particle only went through one hole. Imagine you're up on a ladder, dropping beanbags through a plank with two slits in it (you can cover those slits if you want), and they form a pile on the ground below. If the beanbags can only go through one slit, the pile you get on the ground is a nice mound. If you open up BOTH slits, then what you expect is TWO mounds. If the slits are close enough together, you expect those mounds to overlap, with the height at each spot being AT LEAST AS HIGH as the height you'd see dropping the beanbags through just one hole.
But instead, what you see in the double-slit experiment is that, in between the two mounts, you get spots where there are FEWER beanbags than you'd get dropping them through just one hole. Somehow, instead of getting that 1+1=2, you're finding that 1+1=0. The beanbags are all still there--it's not like they're cancelling each other out.. they're just not all where you'd expect them.
The ONLY WAY to explain this (that we've found so far) is if each beanbag, which, again, you're dropping one at a time, somehow goes through BOTH slits and INTERFERES WITH ITSELF. This is where the idea of wave-particle duality comes in, because the patterns that you see (with valleys where there should be ridges) are similar to what you'd see with water waves or sound waves (sound waves can cancel each other out--that's the whole premise behind noise-cancelling headphones).
So then why don't we just say that photons (and beanbags) are waves and not particles at all? Well, because classical waves aren't "quantal," meaning you can't divide sound waves into discrete, indivisible components. You can have one "particle" of light (a photon). There's no corresponding discrete element of sound. So we say that they're particles after all, and simply adjust our thinking regarding just what a particle is and how one behaves.
I just had a brief look at the published version of the paper. Unless you work on fundamental aspects of quantum information theory, the actual implication is that some old debate that took place back in the 90s has been resolved. As others have already pointed out, the relationship between uncertainty relations and wave-particle duality intuitively makes sense, but actually coming up with a mathematical proof that the two concepts are equivalent to each other is certainly a non-trivial amount of work. However, this paper does not significantly change our understanding of quantum physics, nor does it allow us to magically find an efficient way to simulate quantum physics on classical computers. It will also not change the way quantum physics is usually taught, as wave-particle duality basically plays no role there (and uncertainty relations are mostly a side remark).
Also, notice that the paper has been published in Nature Communications. Usually, this means that the paper was rejected by Nature Physics (or any other of the "Nature Something" journals), so the authors sent it there instead (BTDT). So we probably have at least an editor (and maybe some referees) who thought that the paper was not as sexy as the press release seems to imply.
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Maybe you're thinking of The Feynman Lectures (which is college-level)? In Volume 3, Section 2-2 of his lectures, Feynman shows the deep relationship between the uncertainty principle and wave-particle duality. Feynman sez:
Now this property of waves, that the length of the wave train times the uncertainty of the wave number associated with it is at least 2, is a property that is known to everyone who studies them.
.: Semper Absurda