6 ms·

Is there.. any reason why all protons must be the same size?

by MrLeap 5y ago

stronglikedan 5y ago
layman, but I believe it's the "size" (defined by the wave function) that makes a particle a proton. at least, one of the factors that does, anyway
gowld 5y ago
Protons are made of 3 quarks and a bunch of tiny gluons. if a big proton existed, it would be made of different or more quarks/stuff, and be called something else. Now, is there a reason why there isn't a something else that acts like a proton? That gets into Elementary particle physics and what combinations are stable and have matching charge. Quarks have charge +- n/3 and generally come in triples. Could the number of gluons vary? Maybe? But they wouldn't affect the size measurements much? And variations wouldn't be stable?
db48x 5y ago
Yes. All protons are indistinguishable from each other. No matter what aspect of a proton you measure, it will be identical to any other proton you could measure instead. This has important implications. Consider a quantum mechanics experiment where you emit a proton at A and try to detect it at A′. You will find that that there is some quantifiable chance of detecting the particle at A′ some time T. Call that probability P. Now consider a second experiment where you emit a proton at both A and B, and try to detect them at A′ and B′. What is the probability of finding a proton at A′ at time T? You will find that you get a different number! This is because the proton at A could travel to A′ and be detected, but also the proton at B could travel to A′ and be detected too. Since you cannot distinguish the two protons, you won’t be able to distinguish between these two outcomes, and so the probability must be different from P.
- RobotCaleb 5y ago
  Does this mean that there's possibly only one proton?
  - kazinator 5y ago
    Two protons P0 and P1 are the same only if they match in every single quantum parameter.
  - willis936 5y ago
    On the contrary, protons are baryons and they must each have a unique quantum state (they must be in a unique/distinct location).
  - infogulch 5y ago
    You mean like the "One-electron universe" postulate [1]? First, I don't think it's really useful beyond being a fun idea to consider. Also there's a lot more matter than antimatter, which raises some... logistical problems. Also, unlike electrons/positrons, protons are not fundamental particles, they're made up of quarks which throws a whole other wrench in the idea. [1]: https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe
- busyant 5y ago
  How does your experiment demonstrate that all protons are indistinguishable? Couldn't it be that your measurement for a proton being at A' is simply measuring the wrong feature of the protons? edit: I have no idea if protons are indistinguishable from one another, but this experiment doesn't seem compelling.
  - MrLeap 5y ago
    I was thinking the same thing. If you read the experiment described but replace the word "proton" with "cat" -- I would just assume that the scientists in question were from a society with very coarse senses and measurements, not that all cats are indistinguishable.
    - mcswell 5y ago
      In some versions of object oriented programming, some aspects of objects can be queried from the outside, some cannot--they are invisible states. For protons, they may come with a serial number, but we're not allowed to read it because of the constraints built into God's programming language.
    - arbitrage 5y ago
      It's a mistake to try to make analogies of quantum interactions with everyday objects like cats. It's not as useful as one might hope, and your intuition gets in the way. Protons don't behave like cats.
  - xwolfi 5y ago
    Turn it the other way around: if it has a different level of energy in its quantum field, it doesnt expose the properties of what we call "a proton".
  - scott_s 5y ago
    Think of it this way: we can experimentally figure out what the probabilities are; they are an observed thing. The only way to make sense of these probabilities is if all protons are indistinguishable. In my statistical mechanics course, we went through an illuminating exercise where we started by trying to take account of every atom in a gas cloud. We started taking limits and making assumptions. One of them was that all atoms are indistinguishable from each other. This decreases the possible states of the system by N! (factorial, not surprise). After making that assumption, out pops the ideal gas law.
- 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.
- whatshisface 5y ago
  >and so the probability must be different from P. Not if they interfere in just the right way to keep the probability the same as in P. I see a second problem with this experiment - in the double slit experiment, there is one electron interfering with itself. If you released two protons at the same time, they'd interact with each other and change the probabilities, even classically.
uoaei 5y ago
Each solution to the Schrodinger equation describes a different kind of particle. We assume that QFT is deterministic and complete so each particle will have exactly the same properties as the others of its kind aside from position and momentum (as far as we can tell). As a fun consequence, the theory treats all particles the same -- even so-called quasi-particles! The difference between fundamental and other kinds of particles is that the fundamental particles can exist (at least for a short time before decaying) in vacuo.
- whatshisface 5y ago
  There is no telling whether anything exists in vacuo or merely as excitations in a lattice we're living inside unaware. :)
burnished 5y ago
Good question. I'm not sure. We treat them like they are all the size, which I assume is a function of all them being themselves composed of pieces that we treat as fungible. On the other hand its not like anyone is measuring a proton size with a pair of calipers, so them all being the same 'size' could simply be a function of them all having the same charge.

Is there.. any reason why all protons must be the same size?

by MrLeap 5y ago

Is there.. any reason why all protons must be the same size?

layman, but I believe it's the "size" (defined by the wave function) that makes a particle a proton. at least, one of the factors that does, anyway

gowld 5y ago

Protons are made of 3 quarks and a bunch of tiny gluons. if a big proton existed, it would be made of different or more quarks/stuff, and be called something else. Now, is there a reason why there isn't a something else that acts like a proton? That gets into Elementary particle physics and what combinations are stable and have matching charge. Quarks have charge +- n/3 and generally come in triples. Could the number of gluons vary? Maybe? But they wouldn't affect the size measurements much? And variations wouldn't be stable?

db48x 5y ago

Yes. All protons are indistinguishable from each other. No matter what aspect of a proton you measure, it will be identical to any other proton you could measure instead. This has important implications. Consider a quantum mechanics experiment where you emit a proton at A and try to detect it at A′. You will find that that there is some quantifiable chance of detecting the particle at A′ some time T. Call that probability P. Now consider a second experiment where you emit a proton at both A and B, and try to detect them at A′ and B′. What is the probability of finding a proton at A′ at time T? You will find that you get a different number! This is because the proton at A could travel to A′ and be detected, but also the proton at B could travel to A′ and be detected too. Since you cannot distinguish the two protons, you won’t be able to distinguish between these two outcomes, and so the probability must be different from P.

RobotCaleb 5y ago
Does this mean that there's possibly only one proton?
- kazinator 5y ago
  Two protons P0 and P1 are the same only if they match in every single quantum parameter.
- willis936 5y ago
  On the contrary, protons are baryons and they must each have a unique quantum state (they must be in a unique/distinct location).
- infogulch 5y ago
  You mean like the "One-electron universe" postulate [1]? First, I don't think it's really useful beyond being a fun idea to consider. Also there's a lot more matter than antimatter, which raises some... logistical problems. Also, unlike electrons/positrons, protons are not fundamental particles, they're made up of quarks which throws a whole other wrench in the idea. [1]: https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe
busyant 5y ago
How does your experiment demonstrate that all protons are indistinguishable? Couldn't it be that your measurement for a proton being at A' is simply measuring the wrong feature of the protons? edit: I have no idea if protons are indistinguishable from one another, but this experiment doesn't seem compelling.
- MrLeap 5y ago
  I was thinking the same thing. If you read the experiment described but replace the word "proton" with "cat" -- I would just assume that the scientists in question were from a society with very coarse senses and measurements, not that all cats are indistinguishable.
  - mcswell 5y ago
    In some versions of object oriented programming, some aspects of objects can be queried from the outside, some cannot--they are invisible states. For protons, they may come with a serial number, but we're not allowed to read it because of the constraints built into God's programming language.
  - arbitrage 5y ago
    It's a mistake to try to make analogies of quantum interactions with everyday objects like cats. It's not as useful as one might hope, and your intuition gets in the way. Protons don't behave like cats.
- xwolfi 5y ago
  Turn it the other way around: if it has a different level of energy in its quantum field, it doesnt expose the properties of what we call "a proton".
- scott_s 5y ago
  Think of it this way: we can experimentally figure out what the probabilities are; they are an observed thing. The only way to make sense of these probabilities is if all protons are indistinguishable. In my statistical mechanics course, we went through an illuminating exercise where we started by trying to take account of every atom in a gas cloud. We started taking limits and making assumptions. One of them was that all atoms are indistinguishable from each other. This decreases the possible states of the system by N! (factorial, not surprise). After making that assumption, out pops the ideal gas law.
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.
whatshisface 5y ago
>and so the probability must be different from P. Not if they interfere in just the right way to keep the probability the same as in P. I see a second problem with this experiment - in the double slit experiment, there is one electron interfering with itself. If you released two protons at the same time, they'd interact with each other and change the probabilities, even classically.

uoaei 5y ago

Each solution to the Schrodinger equation describes a different kind of particle. We assume that QFT is deterministic and complete so each particle will have exactly the same properties as the others of its kind aside from position and momentum (as far as we can tell). As a fun consequence, the theory treats all particles the same -- even so-called quasi-particles! The difference between fundamental and other kinds of particles is that the fundamental particles can exist (at least for a short time before decaying) in vacuo.

whatshisface 5y ago
There is no telling whether anything exists in vacuo or merely as excitations in a lattice we're living inside unaware. :)

burnished 5y ago

Good question. I'm not sure. We treat them like they are all the size, which I assume is a function of all them being themselves composed of pieces that we treat as fungible. On the other hand its not like anyone is measuring a proton size with a pair of calipers, so them all being the same 'size' could simply be a function of them all having the same charge.