How Quantum Mechanics produces REALITY & perhaps ARROW of TIME | wave collapse & Decoherence
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2021-05-22に共有
Citations:
Yale research on recording wave collapse, Minev et al: t.ly/cX9v
Bouchard et al paper on observing recoherence: t.ly/rOqb
Quantum mechanics link to time, Smolin et al: t.ly/2Psj
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Chapters:
0:00 - What's the problem?
1:08 - Quantum indeterminism
2:42 - What is superposition?
3:50 - How do we know this is reality?
5:03 - Switch from quantum to classical
6:06 - Quantum decoherence explained
8:52 - Is decoherence really irreversible?
11:26 - Is there a point of no return?
12:18 - How does QM determine time
Summary:
How does the indeterminate world of quantum mechanics, where the future isn’t fixed, become the classical predictable real world we experience? Quantum researchers argue about it even today. It's really all about time, and the boundary between the past and future.
Quantum mechanics insist that the spin of particle has neither one direction or the other until we look. The very act of looking forces the universe to make a choice.
All the variables that characterize the properties of an elementary particles, such as its position and momentum, are encoded in a mathematical expression called a wave function. It is just a sum of the states that the particle could be found in. This is called a superposition.
By itself, the wave function doesn’t have any intuitive meaning. But the square of the wave function gives us the probability of finding that quantum object in any particular place. Prior to measurement, quantum mechanics doesn’t really tell us anything about a particle. We know this is true because particles interfere with each other prior to measurement as in the double slit experiment.
In our macro world, we never see this superposition. So quantum physicists have to add in this transition from quantum to classical step by hand, as something extra to quantum mechanics, ThisIt is often called the “collapse” of the wave function, and was first introduced by the Hungarian mathematical physicist John von Neumann.
A better description of this is decoherence. This is how it works: The quantum state of a particle becomes mixed up – or entangled – with the states of the atoms in the environment. If the particle is in a superposition, this superposition then spreads to the atoms it interacts with. The come these entangled particles interact with, the farther this entanglement spreads until it spreads all across the universe. This leads to decoherence, because not the original particle becomes unrecognizable.
But the problem with decoherence is that in theory there is no end tot he entanglement or spreading of this quantumness. So in principle the superposed state of the original particle could be reconstituted if we could keep track of all the entanglement of all the particles.
But this would be like reversing time. Experiments in “recoherence” has been done however. This would mean undoing the measurement, and erasing any information we gained about it.
So here’s the question: Is there ever a point where the measurement process switches from being fully reversible in principle by this sort of recoherence, and becomes irreversible? Is there a point of no return?
Quantum experts don’t agree about this. As long as the process is reversible, it’s still quantum. It’s only when it becomes irreversible that it becomes classical – and it’s only then that we can truly say “what happened” - when reality becomes real.
Physicist Lee Smolin has suggested that this is in fact what distinguishes the past from the present. The past is classical. The present is quantum. What separates them is whether they are reversible or not, whether they have become known or not. Quantum mechanics may link knowledge, information, and time.
According to Smolin, It’s the change from a quantum, indefinite present to a classical, definite past that defines the very arrow of time itself, pointing it always in the forwardin one direction as the quantum present constantly churns out a classical past.
The past goes away. It is already known. But the future is all possibility. We can never go back – we can’t change the past. In fact, the past no longer really exists at all. “Once something is definite”, says Smolin, “its job is done and it is gone.”
#decoherence
#quantummechanics
But what happens to all those futures that aren’t selected by the decohering present? Where do those alternative possible futures go? Do they pop up in an alternative universe? Or is there just one reality?
Outro Artist of the week: Sander Kalmeijer, performing title "Gold Medal"
コメント (21)
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Here is a follow up video explaining ENTANGLEMENT more fully: https://youtu.be/unb_yoj1Usk Also, here is more detail on what a wave function "collapse" probably is: The whole process of “wave function collapse” proposed by von Neumann effectively introduced, in an ad hoc way - through measurement - a way to make the quantum evolution of a system become irreversible, i.e., to draw a line between quantum and classical. Does this really happen? Probably not. Decoherence is the modern approach to explain how quantum systems transition to classical. It results when a quantum system entangles with its environment. When this happens, the components of the wave function of our original system are decoupled from a coherent system, and they acquire phases from their immediate surroundings. This entanglement spreads not only to the local environment of the quantum system, but everywhere in the universe. Strictly speaking, there is no mechanism in quantum mechanics that “breaks” the entanglement. It keeps spreading. The coherence of our original system becomes unrecognizable. This is analogous to dropping a blob of ink into the ocean. The drop is never formally lost because in principle you could bring all the particles of the ink back together again. In practice that is not possible. Similarly, the original superposition of our initial quantum system, in principle, could be recovered if we could keep track of all the particles and their effect on the phase changes of our original system. But this quickly becomes impossible to do. There is no wave function collapse, but at some point, the original system becomes irretrievable, so there appears to be an "apparent" wave function collapse. Exactly when this happens is what Lee Smolin believes demarcates the present from the past. For details on Smolin's paper, see link in the description.
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I really like how he talks about papers that are published very recently. This shows he studies so much about the things he create videos about.
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Real captions/subtitles/text ! And in 9 languages!!! That's solidarity and professionalism hand-in-hand. You simply cannot know how much it's appreciated!
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This is the most intuitive explanation of quantum mechanics available anywhere. Period. On behalf of the entire layman science enthusiast community, thank you, Arvin. 😌
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Arvin, I just wanted to tell you that, not only is your content just good, but you are by far the best I have encountered at explaining physics. As an older engineering student who's mind isn't as flexible as it was when I was a teenager/early 20's, I can't thank you enough.
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Best way to start the weekend with Arvin's "that's coming up, right now!"
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Arvin Ash, you explain very complicated phenomena in a very simple way that a 72 year old man like me can even grasp. Thanks and keep going.
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I had to watch it more than once, but this is one of the best videos that explains quantum to classical jump. Thank you very much for taking the efforts to create this beauty
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"We live in the borderland" This quote says it all for me! It means we live in the present. This for me describes the borderland of the wave/particle duality. The past is the particle and the future is the wave. The present is the instant when the particle and wave are one in the same. It's bcus we live in this borderland, that we can demonstrate experiments showing waves and particles just based on observation alone. This is why it makes sense that a wave/particle paradox should be the reality for any observer, or to be more accurate, for any measurement of any happening in the universe, even if there is no observer. Slit experiments showing wave patterns are showing us the possible outcomes of what was the future at the moment when we fired the electrons. Slit experiments showing us the double line patterns are showing us the determined outcomes of what became the past when it got measured, meaning the electron's measurement is executed 'before' it hits the photographic plate or in some cases, a computer screen. That's why it doesn't seem to matter if we measure what happened before or after the slits of an apparatus, bcus the measurement is situated to happen before the photographic plate. So, the outcomes are displayed on the photographic plate as particle double line patterns. It should be then, that it only matters if we measure what happened 'after' the electrons strike the photographic plate in the apparatus, to see a wave pattern that is. This is bcus a probability wave when being measured, is equivalent to a probability being collapsed into the sureness of the past, which is the "present" relative to the observer who measured it. The present is indeed relative as shown in relativity theory, so a probability wave which has collapsed into particle form relative to my own observation, may still be a probability wave relative to another observer who has not yet measured the happening. Disagreement on simultaneity means that what is just a probability relative the latter observer, is actually a certainty relative to myself, since I've already collapsed the wave. For me personally, this means that probability waves must be deterministic.(I know I know, bear with me) We can't agree on simultaneity of 2 lightning strikes hitting a train. Suppose I witness the lightning bolts strike at the same time, and yu witness the traincar in the front get struck first, followed by the traincar in the back since you're traveling so fast(we all know this scenario)... Then I am certain of the determined outcome, that the back of the train has been definitely struck. Yet you are still unaware of the back strike. You only see a probability of the back also getting struck, which represents a probability wave relative to you. But relative to me, I know for sure, this back strike is a certainty. So even though you see only a probability wave and you have no possible method to determine with certainty that a 2nd strike will happen, it is still true, even for the unknowing observer, that the 2nd strike is an unavoidable certainty. Therefoee, probability waves must be deterministic too, am I wrong?
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my mouse click seems to be entangled with the like button on this video
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Excellent, Arvin, excellent. Coherence vs decoherence; Quantum vs classical; Present vs time; Aloneness vs interactions; Determined vs uncertainty; Probability vs actuality; Being vs Becoming; all of them features & ingredients in the SALAD OF CHANGE!
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My favorite channel on this app. Definitely helped me ground myself mentally during my quantum physics course. You make mapping these concepts in the head so much easier!
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You describe it so clearly i dont know how better anyone can explain this concept. Thanks Arvin
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This channel is AWESOME!!!!
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You made real one of your finest videos, my friend. Thank you for this masterpiece.
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I can really thank you Arvin, since you finally fully explained me, way more clearly, the “wave function collapse” and this finally gets me to a more reasonable explanation between past & future. Thanks again, you're a master in teaching such complicated things!
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This is the best explanation involving the concepts around the arrow of time I’ve found so far. Thank you!
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Recoherence sounds like a vehicle for reversible computing: that is, doing computing, getting the information, then somehow undoing the computation (in effect resetting the entropic state to an earlier point) while keeping the information. It's one of the holy grails.
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Time, what it actually is, and the Arrow of Time, are such deeply profound and perplexing questions. I doubt any serious science lover does not spend considerable "time" trying to establish a finger-hold on how to properly understand it. This video offered a wonderful additional way to think about time, the past, present and future, in terms of that other great mystery, Quantum Mechanics. That there might be a fundamental and intimate relationship that can better help us characterize time, beyond the 2nd law of Thermodynamics, is exciting. It is interesting, and perhaps more than just a coincidence, that both QM and the 2nd law are probabilistic in nature.
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The best presentation I have ever seen for this specific topic - speaking from my heart! I share your arguments and interpretations. Thank you!
