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Wave particle duality used to stress me out before I read Feynman talk about it. "Things on a very small scale behave like nothing that you have any direct exp

Wave particle duality used to stress me out before I read Feynman talk about it. "Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen..... There is one lucky break, however—electrons behave just like light. The quantum behavior of atomic objects (electrons, protons, neutrons, photons, and so on) is the same for all, they are all “particle waves,” or whatever you want to call them. So what we learn about the properties of electrons (which we shall use for our examples) will apply also to all “particles,” including photons of light."

effie 6y ago
Single beam of electrons diffracts like single beam of light, but otherwise electrons behave very differently than light: two electron beams interfere with each other, while two light beams do not.
- beojan 6y ago
  Two coherent beams of light will absolutely interfere.
  - danbruc 6y ago
    I think that was meant to mean that the two beams interact. Two crossing light beams will pass right through each other without much happening while two electron beams will interact. But this is because electrons are charged and photons are not. On careful inspection and with high energy photons one would also notice an interaction between two light beams due to photon photon scattering [1]. But one could reasonably call this a kind of distraction due to different charges of different particles. Ignoring those makes photons and electrons behave identical in respect to the phenomena discussed in the article. [1] https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics
  - ahartmetz 6y ago
    Electron beams interfere in ways that light beams don't, though. If they cross in vacuum, they deflect each other due to the electric charge carried by electrons. Light beams can only deflect each other in media with non-linear response to electromagnetic fields, i.e. the response to one beam modifies the response to the other beam.
    - klodolph 6y ago
      Technically speaking even the vacuum is nonlinear, according to current theories (although it hasn’t been observed yet).
- jjbinx007 6y ago
  Am I right that thinking of everything as waves isn't a bad mental model? If everything is a field, then light is simply a three dimensional wave in that field. This makes the double slit experiment seem much easier to grasp: A single photon (which is a wave with a discreet amount of energy) travels through both slits at the same time. This causes the wave to interfere with itself and when it hits the wall it registers as a single point, as the photon must always be a discreet quantum of energy. To me, this makes intuitive sense and removes a lot of the mystery and confusion that gets invoked whenever pop-sci writers explain the experiment. They tend to say something along the lines of: photons are like billiard balls, so they can only go through one slit or the other, but if you don't detect which slit it goes through it seems to go through both. Am I close to what's actually going on, or am I way off?
  - harshitaneja 6y ago
    The problem with treating everything as a wave is that all these entities can only exist at certain energies. So the field is quantized.
    - Chris2048 6y ago
      IS quantisation a property that waves cannot have?
  - klodolph 6y ago
    If you want to think about it, I suggest that you keep a “library” of classic experiments in your head. There are experiments that show that light is not a particle (in the classical sense) and other experiments that show that light is not a wave (in the classical sense). Quantum mechanics is the result of trying to find a theory consistent with both sets of experiments. Originally, the double slit experiment was seen as proof that the wave theory is correct. So if you want to challenge your wave theory, look to different experiments. Wave theory of light was predominant from the mid-19th century to the emergence of quantum mechanics in the early 20th century, so I would focus on science from the early 20th century, such as the ultraviolet catastrophe and the photoelectric effect. Consider the photoelectric effect. If light were a wave that carried energy, this wouldn’t explain why an equal amount of energy has a different effect depending on its wavelength. The quantum explanation is that one packet of light contains a different amount of energy, which depends on wavelength. “Light is a wave” kind of falls apart as a theory, because it is unable to explain this.
- simonh 6y ago
  Well yes one has charge and the other doesn't, they aren't literally the same thing so are different QED. The point is the rules are the same.
- quchen 6y ago
  > while two light beams do not [interact] Photons do interact! There is no two-photon vertex, that’s true, since there is no fundamental interaction between two photons. But put four photon/charged particle vertices together and you’ll get the 4-photon box diagram [1], whose contribution is nonzero. [1]: Since I’m not good at painting diagrams in ASCII on HN, here’s a link to the Wikipedia article with the box rendered nicely: https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics
  - ssivark 6y ago
    The propagator is a two photon vertex... interpreted as one in, one out, moving through time :-) Maybe you meant there’s no 2–>1 vertex.
  - effie 6y ago
    That's a prediction. Evidence is so far inconclusive.
l33tman 6y ago
Just to add a nitpick here, apart from them sharing some wave-like descriptions at some levels in the theory, there is not that much in common between photons and electrons. Fundamentally they play different parts in the theory (one being a matter particle the other a force carrier) and mathematically they are represented differently and behaves very differently (one being a fermion and the other a boson). In particular the boson/fermion distinction is usually glossed over in popular treatments of QED (because it's so unintuitive and messy).
- jiggawatts 6y ago
  Meanwhile, in a very real sense the following equation holds: electron + positron = 2x photon So it's somewhat (very?) false to say that they're fundamentally distinct, when both leptons and photons are fundamental components of electromagnetic phenomena, and are even interconvertible! If you squint hard enough, it looks like leptons behave like sufficiently energetic photons self-interacting to the point that they form a localised circulation. This requires a charge separation to be stable, hence the requirement for a pair of oppositely charged leptons to be formed from a photon.
jiggawatts 6y ago
Don't try to understand transistors in concepts you're familiar with, such as cars and bananas. A transistor is neither a car, nor a banana. You have to have a new kind of understanding, one that transcends such mundane, familiar concepts. Get it? Feynman explained nothing by saying that particles are "not like" two other familiar concepts. That's flipping two bits in an infinite set of to "false". You can't gain understanding by flipping all but one remaining bit in that infinite set to false! Particles aren't like tiny springs either, or tiny metronomes, or tiny bolts of lightning, or... an infinite list of things they aren't! Similarly, Feynman's Path Integral method is trotted out as a mental model of how particles "work" at the Quantum Level, but this is literally just a mathematical trick for solving a class of problems efficiently. This is not my opinion, Feynman said so. It's not a "model of the world", it is literally just a specific case of Monte Carlo Integration!

effie 6y ago

Single beam of electrons diffracts like single beam of light, but otherwise electrons behave very differently than light: two electron beams interfere with each other, while two light beams do not.

beojan 6y ago
Two coherent beams of light will absolutely interfere.
- danbruc 6y ago
  I think that was meant to mean that the two beams interact. Two crossing light beams will pass right through each other without much happening while two electron beams will interact. But this is because electrons are charged and photons are not. On careful inspection and with high energy photons one would also notice an interaction between two light beams due to photon photon scattering [1]. But one could reasonably call this a kind of distraction due to different charges of different particles. Ignoring those makes photons and electrons behave identical in respect to the phenomena discussed in the article. [1] https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics
- ahartmetz 6y ago
  Electron beams interfere in ways that light beams don't, though. If they cross in vacuum, they deflect each other due to the electric charge carried by electrons. Light beams can only deflect each other in media with non-linear response to electromagnetic fields, i.e. the response to one beam modifies the response to the other beam.
  - klodolph 6y ago
    Technically speaking even the vacuum is nonlinear, according to current theories (although it hasn’t been observed yet).
jjbinx007 6y ago
Am I right that thinking of everything as waves isn't a bad mental model? If everything is a field, then light is simply a three dimensional wave in that field. This makes the double slit experiment seem much easier to grasp: A single photon (which is a wave with a discreet amount of energy) travels through both slits at the same time. This causes the wave to interfere with itself and when it hits the wall it registers as a single point, as the photon must always be a discreet quantum of energy. To me, this makes intuitive sense and removes a lot of the mystery and confusion that gets invoked whenever pop-sci writers explain the experiment. They tend to say something along the lines of: photons are like billiard balls, so they can only go through one slit or the other, but if you don't detect which slit it goes through it seems to go through both. Am I close to what's actually going on, or am I way off?
- harshitaneja 6y ago
  The problem with treating everything as a wave is that all these entities can only exist at certain energies. So the field is quantized.
  - Chris2048 6y ago
    IS quantisation a property that waves cannot have?
- klodolph 6y ago
  If you want to think about it, I suggest that you keep a “library” of classic experiments in your head. There are experiments that show that light is not a particle (in the classical sense) and other experiments that show that light is not a wave (in the classical sense). Quantum mechanics is the result of trying to find a theory consistent with both sets of experiments. Originally, the double slit experiment was seen as proof that the wave theory is correct. So if you want to challenge your wave theory, look to different experiments. Wave theory of light was predominant from the mid-19th century to the emergence of quantum mechanics in the early 20th century, so I would focus on science from the early 20th century, such as the ultraviolet catastrophe and the photoelectric effect. Consider the photoelectric effect. If light were a wave that carried energy, this wouldn’t explain why an equal amount of energy has a different effect depending on its wavelength. The quantum explanation is that one packet of light contains a different amount of energy, which depends on wavelength. “Light is a wave” kind of falls apart as a theory, because it is unable to explain this.
simonh 6y ago
Well yes one has charge and the other doesn't, they aren't literally the same thing so are different QED. The point is the rules are the same.
quchen 6y ago
> while two light beams do not [interact] Photons do interact! There is no two-photon vertex, that’s true, since there is no fundamental interaction between two photons. But put four photon/charged particle vertices together and you’ll get the 4-photon box diagram [1], whose contribution is nonzero. [1]: Since I’m not good at painting diagrams in ASCII on HN, here’s a link to the Wikipedia article with the box rendered nicely: https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics
- ssivark 6y ago
  The propagator is a two photon vertex... interpreted as one in, one out, moving through time :-) Maybe you meant there’s no 2–>1 vertex.
- effie 6y ago
  That's a prediction. Evidence is so far inconclusive.

l33tman 6y ago

Just to add a nitpick here, apart from them sharing some wave-like descriptions at some levels in the theory, there is not that much in common between photons and electrons. Fundamentally they play different parts in the theory (one being a matter particle the other a force carrier) and mathematically they are represented differently and behaves very differently (one being a fermion and the other a boson). In particular the boson/fermion distinction is usually glossed over in popular treatments of QED (because it's so unintuitive and messy).

jiggawatts 6y ago
Meanwhile, in a very real sense the following equation holds: electron + positron = 2x photon So it's somewhat (very?) false to say that they're fundamentally distinct, when both leptons and photons are fundamental components of electromagnetic phenomena, and are even interconvertible! If you squint hard enough, it looks like leptons behave like sufficiently energetic photons self-interacting to the point that they form a localised circulation. This requires a charge separation to be stable, hence the requirement for a pair of oppositely charged leptons to be formed from a photon.

jiggawatts 6y ago

Don't try to understand transistors in concepts you're familiar with, such as cars and bananas. A transistor is neither a car, nor a banana. You have to have a new kind of understanding, one that transcends such mundane, familiar concepts. Get it? Feynman explained nothing by saying that particles are "not like" two other familiar concepts. That's flipping two bits in an infinite set of to "false". You can't gain understanding by flipping all but one remaining bit in that infinite set to false! Particles aren't like tiny springs either, or tiny metronomes, or tiny bolts of lightning, or... an infinite list of things they aren't! Similarly, Feynman's Path Integral method is trotted out as a mental model of how particles "work" at the Quantum Level, but this is literally just a mathematical trick for solving a class of problems efficiently. This is not my opinion, Feynman said so. It's not a "model of the world", it is literally just a specific case of Monte Carlo Integration!