4 ms·

ELI5 how it's different?

by kolinko 11y ago

dhimes 11y ago
"ELI5" is "Explain like I'm 5" in case anybody is wondering.
themgt 11y ago
The actual description of "how it's different" in QM is going to depend on the interpretation you choose, but classical vs. QM is basically the difference between "I don't know yet" vs. "it isn't yet"
- hohenheim 11y ago
  To elaborate a bit, Copenhagen Interpretation of QM states that the cat is both alive and dead. It will only settle down on a form, dead or alive, once we observe it. You could read farther on the subject [1]. [1] https://en.wikipedia.org/wiki/Copenhagen_interpretation#Consequences https://en.wikipedia.org/wiki/Copenhagen_interpretation#Cons...
cpd85 11y ago
Sure! One of the major advantages of quantum computing is that you can act on the superposition, i.e. construct some sort of operation which goes "Map the dead cat state to x, and the live cat state to y." This becomes useful when you can generate a superposition of all possibilities, and then perform operations which make it more probable to see the states you desire. In the classical version, the final result of mapping the dead cat would be only x or y
tubelite 11y ago
This is how I think about it (don't take my word for it, I don't really understand QM) QM is like lazy evaluation. The result is not computed until someone actually forces it. This is different from the result being computed but you just don't know what it is. Another way is to think of entangled states is like two virtual addresses which alias to the same physical address. The addresses could be very far apart (similar to entangled particles being very far apart), and their contents are coupled but "uncomputed", until one of the virtual addresses is accessed, faulting in the physical page. At this point, both addresses instantly point to the same contents.
- arrel 11y ago
  As a programmer who only vaguely understands QM, the lazy evaluation example is amazing. Any physicists want to confirm its right?
  - blincoln 11y ago
    IANAP, but I think using "lazy evaluation" as an analogy is missing one of the most interesting aspects of quantum physics: it's not just that the particles (or what have you) are in an indeterminate state. The fact that they're in an indeterminate state changes the way they behave. The reason it's possible to build up interference patterns using single electrons is because the various probabilities for its path between the electron gun and target are interfering with each other. I'm pretty sure a typical "lazy evaluation" system will not exhibit that type of behaviour :).
  - colanderman 11y ago
    It's not. When you're in a superposition of two states, it only appears to be "indeterminate" under measurements which are not orthogonal to the state. To measurements which are orthogonal, the state is 100% determinate. What's more, measuring with a non-orthogonal measurement makes the state orthogonal to that measurement, and not orthogonal to the other measurement! This is totally different to how lazy evaluation works.
  - Jach 11y ago
    I think lazy eval is the wrong way to think about it. You might like http://oyhus.no/QuantumMechanicsForProgrammers.html http://oyhus.no/QuantumMechanicsForProgrammers.html
static_noise 11y ago
Consider the cat in the box. Is it alredy dead OR alive (classical interpretation) or is it both at the same time (quantum mechanics) before you open the box and look? The classical interpretation needs so-called hidden variables which determine the state of the cat. In the quantum mechanics interpretation the state is determined when the interaction (measurement, looking at it) takes place. Before that there are several parallel realities of what could be. In the case of the cat we cannot actually determine which interpretation is correct. We need a different kind of experiment: As far as I understood (no physicist here) they use two entangled objects. E.g. two photons which are generated by a special process so that one polarized one way and the other one polarized the other way but we don't know which is which before the measurement. Then you need to measure both photons separately and determine (statistically) which has which polarization. These measurements need to be spaced out far enough such that faster than light speed communication would be required for the photon measured first to tell the second photon what was happening. So far it still doesn't help to exclude the possibility of hidden variables since the measurement setup is static. Another trick is needed. They use a random generator to determine at both ends what to measure (which polarization direction) shortly before doing the actual measurement. So, when the photon is generated it doesn't know what would be acutally measured. When you tabularize the all the possible combinations of how the photons could be measured and calculate the possible probability for the measurement results, there is a difference between what we would expect from classical physics when the status is determined at photon generation and stored inside the photons in a hidden variable and the result we would get from quantum mechanics without a hidden variable but with (faster than light) spooky entanglements. The experiments show that there is no hidden variable. This is a BIG thing because this means that classical physics are not enough to explain the universe.
Consultant32452 11y ago
The video on this page blew my mind. Hope it helps you understand how it's actually all states at once. http://themindunleashed.org/2015/06/new-mind-blowing-experiment-confirms-that-reality-doesnt-exist-if-you-are-not-looking-at-it.html http://themindunleashed.org/2015/06/new-mind-blowing-experim...
fricken4 11y ago
This documentary does the best ELI5 of the double slit experiment and how it has confounded physicists that I've seen: https://vimeo.com/126833477 https://vimeo.com/126833477
ColinDabritz 11y ago
The basic cat scenario isn't "interesting" yet, you're right, it's just "we don't know." Importantly you would expect this result even if the cat was observed. You'd still observe a dead cat at the end, or chances at a live cat at different points during the experiment. (The original thought experiment killed the cat randomly over a time span using radioactive decay to release poison randomly during the time). The science part here, is that a hypothesis has been proposed about the inner workings, and the "so what?" objection is a valid question. If the "so what?" is nothing, it's just an idea. The "so what?" part is where the science happens. In trying to promote the idea (hypothesis) as a working idea with evidence (theory) We look for side effects of the idea that are expected to be different than our existing ideas (theories). We expect to see a cat when we open the box, and it's alive or dead. That's not interesting. But let's actually ask a science-like question of the consequences. These aren't related to the quantum mechanics questions really, but it's the sort of "consequences" questions that do get asked. Maybe we can do the experiment over a longer time, like a week. We put the cat in the box with food and water. In the original experiment the poison is released at a random time during the week. So under our existing understanding (theory) we expect, if we run the experiment a hundred times we would see that random amounts of food and water are left pretty much evenly from 0% to 100% depending on when the cat randomly actually died. Here is a different idea (hypothesis) about the side effects: If the cat really is alive and dead as hypothesized, it will use only the part of the food the 'alive' part would use, which is reasonably predictable. It changes by percentage over the week from 100% (alive) to 0% (fully dead). This means we should always see the same, or similar amounts of food and water left at the end. This is different than the usual prediction! Now we have a new idea, a hypothesis that relies on consequences of the first hypothesis. So we run 100 experiments, and unlike our current understanding of cat mortality, we find that in this case the remaining food and water values a grouped up at about 50%! We can even do a control where the cat is observed the entire week, and we find that under these "observed" conditions our expected spread of values does happen. We've proven that our previous idea isn't enough to explain everything, and we have tested a prediction of the new idea, so our previous theory isn't "wrong" but it isn't entirely accurate, and the new hypothesis has gained evidence to become a new more complete theory. Now in the real world at the cat scale, we definitely would NOT expect this to be the case, and real science is much more complicated. It's easy to make errors in predictions, or have "confounding" factors that throw off our investigations. But the initial investigations into spooky action were every bit as bizarre and unexpected. We're learning how things work at the atomic scale, and it turns out that while they mostly behaved as we though they did, there are some odd edge cases that really turn things on their head. Much like we don't poison cats in our day to day life, just in thought experiments (I hope), we don't interact with spooky action in science a whole lot either. But as we learn more, it informs our understanding, and frequently leads some unexpected concequences. For an excellent real-world consequence, under the classic model, photosythesis is too efficient. It requires quantum entanglement to explain the efficiency! http://io9.com/new-evidence-that-plants-get-their-energy-using-quantum-1498695627 http://io9.com/new-evidence-that-plants-get-their-energy-usi...

ELI5 how it's different?

by kolinko 11y ago

ELI5 how it's different?

dhimes 11y ago

"ELI5" is "Explain like I'm 5" in case anybody is wondering.

themgt 11y ago

The actual description of "how it's different" in QM is going to depend on the interpretation you choose, but classical vs. QM is basically the difference between "I don't know yet" vs. "it isn't yet"

hohenheim 11y ago
To elaborate a bit, Copenhagen Interpretation of QM states that the cat is both alive and dead. It will only settle down on a form, dead or alive, once we observe it. You could read farther on the subject [1]. [1] https://en.wikipedia.org/wiki/Copenhagen_interpretation#Consequences https://en.wikipedia.org/wiki/Copenhagen_interpretation#Cons...

cpd85 11y ago

Sure! One of the major advantages of quantum computing is that you can act on the superposition, i.e. construct some sort of operation which goes "Map the dead cat state to x, and the live cat state to y." This becomes useful when you can generate a superposition of all possibilities, and then perform operations which make it more probable to see the states you desire. In the classical version, the final result of mapping the dead cat would be only x or y

tubelite 11y ago

This is how I think about it (don't take my word for it, I don't really understand QM) QM is like lazy evaluation. The result is not computed until someone actually forces it. This is different from the result being computed but you just don't know what it is. Another way is to think of entangled states is like two virtual addresses which alias to the same physical address. The addresses could be very far apart (similar to entangled particles being very far apart), and their contents are coupled but "uncomputed", until one of the virtual addresses is accessed, faulting in the physical page. At this point, both addresses instantly point to the same contents.

arrel 11y ago
As a programmer who only vaguely understands QM, the lazy evaluation example is amazing. Any physicists want to confirm its right?
- blincoln 11y ago
  IANAP, but I think using "lazy evaluation" as an analogy is missing one of the most interesting aspects of quantum physics: it's not just that the particles (or what have you) are in an indeterminate state. The fact that they're in an indeterminate state changes the way they behave. The reason it's possible to build up interference patterns using single electrons is because the various probabilities for its path between the electron gun and target are interfering with each other. I'm pretty sure a typical "lazy evaluation" system will not exhibit that type of behaviour :).
- colanderman 11y ago
  It's not. When you're in a superposition of two states, it only appears to be "indeterminate" under measurements which are not orthogonal to the state. To measurements which are orthogonal, the state is 100% determinate. What's more, measuring with a non-orthogonal measurement makes the state orthogonal to that measurement, and not orthogonal to the other measurement! This is totally different to how lazy evaluation works.
- Jach 11y ago
  I think lazy eval is the wrong way to think about it. You might like http://oyhus.no/QuantumMechanicsForProgrammers.html http://oyhus.no/QuantumMechanicsForProgrammers.html

static_noise 11y ago

Consider the cat in the box. Is it alredy dead OR alive (classical interpretation) or is it both at the same time (quantum mechanics) before you open the box and look? The classical interpretation needs so-called hidden variables which determine the state of the cat. In the quantum mechanics interpretation the state is determined when the interaction (measurement, looking at it) takes place. Before that there are several parallel realities of what could be. In the case of the cat we cannot actually determine which interpretation is correct. We need a different kind of experiment: As far as I understood (no physicist here) they use two entangled objects. E.g. two photons which are generated by a special process so that one polarized one way and the other one polarized the other way but we don't know which is which before the measurement. Then you need to measure both photons separately and determine (statistically) which has which polarization. These measurements need to be spaced out far enough such that faster than light speed communication would be required for the photon measured first to tell the second photon what was happening. So far it still doesn't help to exclude the possibility of hidden variables since the measurement setup is static. Another trick is needed. They use a random generator to determine at both ends what to measure (which polarization direction) shortly before doing the actual measurement. So, when the photon is generated it doesn't know what would be acutally measured. When you tabularize the all the possible combinations of how the photons could be measured and calculate the possible probability for the measurement results, there is a difference between what we would expect from classical physics when the status is determined at photon generation and stored inside the photons in a hidden variable and the result we would get from quantum mechanics without a hidden variable but with (faster than light) spooky entanglements. The experiments show that there is no hidden variable. This is a BIG thing because this means that classical physics are not enough to explain the universe.

Consultant32452 11y ago

The video on this page blew my mind. Hope it helps you understand how it's actually all states at once. http://themindunleashed.org/2015/06/new-mind-blowing-experiment-confirms-that-reality-doesnt-exist-if-you-are-not-looking-at-it.html http://themindunleashed.org/2015/06/new-mind-blowing-experim...

fricken4 11y ago

This documentary does the best ELI5 of the double slit experiment and how it has confounded physicists that I've seen: https://vimeo.com/126833477 https://vimeo.com/126833477

ColinDabritz 11y ago

The basic cat scenario isn't "interesting" yet, you're right, it's just "we don't know." Importantly you would expect this result even if the cat was observed. You'd still observe a dead cat at the end, or chances at a live cat at different points during the experiment. (The original thought experiment killed the cat randomly over a time span using radioactive decay to release poison randomly during the time). The science part here, is that a hypothesis has been proposed about the inner workings, and the "so what?" objection is a valid question. If the "so what?" is nothing, it's just an idea. The "so what?" part is where the science happens. In trying to promote the idea (hypothesis) as a working idea with evidence (theory) We look for side effects of the idea that are expected to be different than our existing ideas (theories). We expect to see a cat when we open the box, and it's alive or dead. That's not interesting. But let's actually ask a science-like question of the consequences. These aren't related to the quantum mechanics questions really, but it's the sort of "consequences" questions that do get asked. Maybe we can do the experiment over a longer time, like a week. We put the cat in the box with food and water. In the original experiment the poison is released at a random time during the week. So under our existing understanding (theory) we expect, if we run the experiment a hundred times we would see that random amounts of food and water are left pretty much evenly from 0% to 100% depending on when the cat randomly actually died. Here is a different idea (hypothesis) about the side effects: If the cat really is alive and dead as hypothesized, it will use only the part of the food the 'alive' part would use, which is reasonably predictable. It changes by percentage over the week from 100% (alive) to 0% (fully dead). This means we should always see the same, or similar amounts of food and water left at the end. This is different than the usual prediction! Now we have a new idea, a hypothesis that relies on consequences of the first hypothesis. So we run 100 experiments, and unlike our current understanding of cat mortality, we find that in this case the remaining food and water values a grouped up at about 50%! We can even do a control where the cat is observed the entire week, and we find that under these "observed" conditions our expected spread of values does happen. We've proven that our previous idea isn't enough to explain everything, and we have tested a prediction of the new idea, so our previous theory isn't "wrong" but it isn't entirely accurate, and the new hypothesis has gained evidence to become a new more complete theory. Now in the real world at the cat scale, we definitely would NOT expect this to be the case, and real science is much more complicated. It's easy to make errors in predictions, or have "confounding" factors that throw off our investigations. But the initial investigations into spooky action were every bit as bizarre and unexpected. We're learning how things work at the atomic scale, and it turns out that while they mostly behaved as we though they did, there are some odd edge cases that really turn things on their head. Much like we don't poison cats in our day to day life, just in thought experiments (I hope), we don't interact with spooky action in science a whole lot either. But as we learn more, it informs our understanding, and frequently leads some unexpected concequences. For an excellent real-world consequence, under the classic model, photosythesis is too efficient. It requires quantum entanglement to explain the efficiency! http://io9.com/new-evidence-that-plants-get-their-energy-using-quantum-1498695627 http://io9.com/new-evidence-that-plants-get-their-energy-usi...