5 ms·

Your hypothetical seems to describe a current inability to differentiate between protons, rather than convincing me that protons must be identical. Is this like

by MrLeap 5y ago

Your hypothetical seems to describe a current inability to differentiate between protons, rather than convincing me that protons must be identical. Is this like the monsters on old sea maps, just a gate around an unknown?

octoberfranklin 5y ago
There is a lot of circular reasoning involved in quantum information and thermodynamics. It's all totally, perfectly self consistent, but it does not derive from first principles like set theory or mathematical logic do. Physics is an experimental science; they are not required to state their axioms. Oftentimes they do (QFT for example), but the most glaring case where they don't is anything involving information. The whole postulate that information is physical is something that was stumbled upon, and then turned out to explain a whole bunch of other weird things like heat and entropy, and some of those explanations in turn implied that information is physical. I suspect that our current efforts to build cryptographically-relevant quantum computers are a lot like the efforts to build perpetual motion machines in the 1700s. Our current understanding of things isn't wrong, but there is some undiscovered general principle that we keep butting up against, so we'll keep trying to build these things until we figure out why nature keeps blocking us. That discovery -- rather than a computationally-useful device -- will be the most important result of all the quantum computing research going on right now.
- adgjlsfhk1 5y ago
  that seems plausible, but currently very unsupported by evidence. in the past decade, quantum computers have gotten way more powerful, and progress doesn't seem to be stalling out.
- wizzwizz4 5y ago
  > until we figure out why nature keeps blocking us A quantum computer only behaves like our mathematical ideal quantum computer if it's sufficiently isolated from the rest of the universe: otherwise you don't get the “indistinguishable states” thing and the amplitudes don't sum, and the computer stops computing and starts being a regular ol' physics experiment. It's an engineering problem, as far as I understand. Get things cold enough, get things isolated enough, so they stay entangled with each other and not with the rest of the universe.
  - pishpash 5y ago
    Is it? In engineering problems you have to consider practical feasibility. Would it turn out that for the expenditure of energy to get things isolated we may as well spend it on classical computation?
    - wizzwizz4 5y ago
      On Earth? Maybe. In deep space? Probably not.
      - evanb 5y ago
        The cosmic microwave background makes even deep intergalactic space warmer (3 or 4 K) than the inside of Earth’s best fridges (the best as low as nanoK). 3 or 4 K is easy to achieve using liquid helium cooling, which is presumably why eg. Google’s Sycamore chip operators in that range. But achieving those low temperatures is a regular occurrence in labs all over the world—-you call AirGas or somebody and they deliver a dewar of liquid He.
        wizzwizz4 5y ago
        And, close to a star, you have several neutrinos flying through your experiment. You have vibrations from earthquakes. You have electromagnetic coupling. etc, etc. The temperatures are easy enough; you just have to compensate for thermal noise. The isolation isn't; without isolation, your signal is wrong.
        pishpash 5y ago
        But unless you're doing a computation without I/O (i.e. no reading back the results, no providing inputs), you need coupling into the rest of the universe, and typically into a low-entropy part of the universe like the Earth, where you have entities that care about the computation. So, I am not convinced having a deeply isolated part of the universe is the answer; in fact, the isolation vs. signal quality tradeoff makes it sound more and more like a fundamental limitation of practical concern.
Iolaum 5y ago
Think of it in a simpler way. There is nothing we can currently measure, that will be different for one proton versus another.
- deleted 5y ago
  [deleted]
- db48x 5y ago
  No. Bell’s Theorem states that if subatomic particles have any “hidden variable” which we cannot currently measure, then that “hidden variable” will either have no effect at all on the particles, or that it will have to have instantaneous “nonlocal” effects on them. If this hidden variable has no effect at all, then it is useless. Most physicists don’t care to include extra variables in a quantum mechanical theory that have no effect at all. By definition, they could not be measured, and so they would be extra baggage to carry around to no effect. Physicists don’t much like nonlocal theories either. Any theory that requires information about the particle at A to travel instantaneously to B in order to change the state of the particle there is going to be hard to sell. You might have heard of a fellow called Einstein, who proved that nothing can go faster than the speed of light. Thus, most physicists take the easier road, as it requires only that subatomic particles are indistinguishable. All this means is that particles are too simple to be uniquely identified. You can in principle tell the difference between two baseballs, because they have different patterns of wear and other markings on their surface. But those wear patterns and markings are formed out of the complex arrangements of trillions or quadrillions of atoms. It is easy to see how rearranging the ink molecules on the surface of a baseball could create a unique baseball, or how selectively removing molecules from the surface of the leather (by scratching it, for example) could do the same. But subatomic particles are too simple to have that kind of internal state. Even atoms are only slightly distinguishable. Most carbon atoms have 12 electrons, but a carbon ion might have 11 or 13 electrons. You can distinguish between the atom and the ion, but not between two atoms or two ions.
MatteoFrigo 5y ago
I think that GP is trying to say the following. In quantum mechanics, probabilities are given by the square of the absolute value of more fundamental quantities called amplitudes. When something happens in two ways that can be distinguished, you must add the probabilities. When something happens in two indistinguishable ways, you must add the amplitudes, which yields a different probability after squaring. For example, .3^2+.4^2 != (.3+.4)^2. Thus, you can verify experimentally whether particles are or are not distinguishable.
- JumpCrisscross 5y ago
  > you can verify experimentally whether particles are or are not distinguishable Thank you for a terrific explanation. Could you please go one layer deeper? Why does whether probabilities or amplitudes are summed imply fungibility (or its absence)?
  - wizzwizz4 5y ago
    Because you can construct experiments where the proton from source A ends up at location B, and the proton from source C ends up at location D, or A ends up at D and B ends up at C. (Or some other possibilities.) You find that the A→B, C→D possibility's amplitude sums with the A→D, B→C possibility: i.e., that they're the same indistinguishable final state. If swapping two things around gives a result indistinguishable from not swapping them, that's fungibility.
    - whatshisface 5y ago
      Photons of different polarizations are nonidentical, but if this argument is true, it would also prove that horizontally and vertically polarized photons were identical... in an experiment insensitive to polarization. I do not believe this answers the original question.
      - wizzwizz4 5y ago
        Two coins aren't identical, but they're fungible. Is "identical" even well-defined on fundamental-ish particles?
analog31 5y ago
Every description of a scientific idea always carries the implied disclaimer that we are talking about the present state of our knowledge. Every "fact" is in fact a belief that's held on a tentative basis pending the introduction of better evidence. That sounds quite noncommittal, but in fact it's the best that we can hope for. One interesting thing about protons is that you can describe the effect of exchanging any two of them in precise terms, and end up with macroscopic predictions that can be tested. So you're not limited to just trying to measure every proton in a bag to see if they're all the same. There are other ways to test the hypothesis.

by MrLeap 5y ago

octoberfranklin 5y ago

There is a lot of circular reasoning involved in quantum information and thermodynamics. It's all totally, perfectly self consistent, but it does not derive from first principles like set theory or mathematical logic do. Physics is an experimental science; they are not required to state their axioms. Oftentimes they do (QFT for example), but the most glaring case where they don't is anything involving information. The whole postulate that information is physical is something that was stumbled upon, and then turned out to explain a whole bunch of other weird things like heat and entropy, and some of those explanations in turn implied that information is physical. I suspect that our current efforts to build cryptographically-relevant quantum computers are a lot like the efforts to build perpetual motion machines in the 1700s. Our current understanding of things isn't wrong, but there is some undiscovered general principle that we keep butting up against, so we'll keep trying to build these things until we figure out why nature keeps blocking us. That discovery -- rather than a computationally-useful device -- will be the most important result of all the quantum computing research going on right now.

adgjlsfhk1 5y ago
that seems plausible, but currently very unsupported by evidence. in the past decade, quantum computers have gotten way more powerful, and progress doesn't seem to be stalling out.
wizzwizz4 5y ago
> until we figure out why nature keeps blocking us A quantum computer only behaves like our mathematical ideal quantum computer if it's sufficiently isolated from the rest of the universe: otherwise you don't get the “indistinguishable states” thing and the amplitudes don't sum, and the computer stops computing and starts being a regular ol' physics experiment. It's an engineering problem, as far as I understand. Get things cold enough, get things isolated enough, so they stay entangled with each other and not with the rest of the universe.
- pishpash 5y ago
  Is it? In engineering problems you have to consider practical feasibility. Would it turn out that for the expenditure of energy to get things isolated we may as well spend it on classical computation?
  - wizzwizz4 5y ago
    On Earth? Maybe. In deep space? Probably not.
    - evanb 5y ago
      The cosmic microwave background makes even deep intergalactic space warmer (3 or 4 K) than the inside of Earth’s best fridges (the best as low as nanoK). 3 or 4 K is easy to achieve using liquid helium cooling, which is presumably why eg. Google’s Sycamore chip operators in that range. But achieving those low temperatures is a regular occurrence in labs all over the world—-you call AirGas or somebody and they deliver a dewar of liquid He.
      - wizzwizz4 5y ago
        And, close to a star, you have several neutrinos flying through your experiment. You have vibrations from earthquakes. You have electromagnetic coupling. etc, etc. The temperatures are easy enough; you just have to compensate for thermal noise. The isolation isn't; without isolation, your signal is wrong.
        pishpash 5y ago
        But unless you're doing a computation without I/O (i.e. no reading back the results, no providing inputs), you need coupling into the rest of the universe, and typically into a low-entropy part of the universe like the Earth, where you have entities that care about the computation. So, I am not convinced having a deeply isolated part of the universe is the answer; in fact, the isolation vs. signal quality tradeoff makes it sound more and more like a fundamental limitation of practical concern.

Iolaum 5y ago

Think of it in a simpler way. There is nothing we can currently measure, that will be different for one proton versus another.

deleted 5y ago
[deleted]
db48x 5y ago
No. Bell’s Theorem states that if subatomic particles have any “hidden variable” which we cannot currently measure, then that “hidden variable” will either have no effect at all on the particles, or that it will have to have instantaneous “nonlocal” effects on them. If this hidden variable has no effect at all, then it is useless. Most physicists don’t care to include extra variables in a quantum mechanical theory that have no effect at all. By definition, they could not be measured, and so they would be extra baggage to carry around to no effect. Physicists don’t much like nonlocal theories either. Any theory that requires information about the particle at A to travel instantaneously to B in order to change the state of the particle there is going to be hard to sell. You might have heard of a fellow called Einstein, who proved that nothing can go faster than the speed of light. Thus, most physicists take the easier road, as it requires only that subatomic particles are indistinguishable. All this means is that particles are too simple to be uniquely identified. You can in principle tell the difference between two baseballs, because they have different patterns of wear and other markings on their surface. But those wear patterns and markings are formed out of the complex arrangements of trillions or quadrillions of atoms. It is easy to see how rearranging the ink molecules on the surface of a baseball could create a unique baseball, or how selectively removing molecules from the surface of the leather (by scratching it, for example) could do the same. But subatomic particles are too simple to have that kind of internal state. Even atoms are only slightly distinguishable. Most carbon atoms have 12 electrons, but a carbon ion might have 11 or 13 electrons. You can distinguish between the atom and the ion, but not between two atoms or two ions.

MatteoFrigo 5y ago

I think that GP is trying to say the following. In quantum mechanics, probabilities are given by the square of the absolute value of more fundamental quantities called amplitudes. When something happens in two ways that can be distinguished, you must add the probabilities. When something happens in two indistinguishable ways, you must add the amplitudes, which yields a different probability after squaring. For example, .3^2+.4^2 != (.3+.4)^2. Thus, you can verify experimentally whether particles are or are not distinguishable.

JumpCrisscross 5y ago
> you can verify experimentally whether particles are or are not distinguishable Thank you for a terrific explanation. Could you please go one layer deeper? Why does whether probabilities or amplitudes are summed imply fungibility (or its absence)?
- wizzwizz4 5y ago
  Because you can construct experiments where the proton from source A ends up at location B, and the proton from source C ends up at location D, or A ends up at D and B ends up at C. (Or some other possibilities.) You find that the A→B, C→D possibility's amplitude sums with the A→D, B→C possibility: i.e., that they're the same indistinguishable final state. If swapping two things around gives a result indistinguishable from not swapping them, that's fungibility.
  - whatshisface 5y ago
    Photons of different polarizations are nonidentical, but if this argument is true, it would also prove that horizontally and vertically polarized photons were identical... in an experiment insensitive to polarization. I do not believe this answers the original question.
    - wizzwizz4 5y ago
      Two coins aren't identical, but they're fungible. Is "identical" even well-defined on fundamental-ish particles?

analog31 5y ago

Every description of a scientific idea always carries the implied disclaimer that we are talking about the present state of our knowledge. Every "fact" is in fact a belief that's held on a tentative basis pending the introduction of better evidence. That sounds quite noncommittal, but in fact it's the best that we can hope for. One interesting thing about protons is that you can describe the effect of exchanging any two of them in precise terms, and end up with macroscopic predictions that can be tested. So you're not limited to just trying to measure every proton in a bag to see if they're all the same. There are other ways to test the hypothesis.