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Is light fundamentally a wave or a particle?

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Re: Is light fundamentally a wave or a particle?

#61

Post by im3w1l »

quietbritishjim wrote: post #60
im3w1l wrote: post #27

In each of the many-worlds it is a particle. The wave behavior is caused by the interaction of the many-worlds.

Does that explain the two slit experiment, with a single particle inferring with its own wave function? If you consider it to be a particle, then you probably need to make the interact of the many-worlds be wavelike. At that point you probably have a more complex model than admitting the photon was not a particle.

> you probably need to make the interact of the many-worlds be wavelike.

Yes

> At that point you probably have a more complex model than admitting the photon was not a particle.

Yes again. But then why a more complex model? To be able to deal with interference of complex systems of entangled particles.

Re: Is light fundamentally a wave or a particle?

#62

Post by ssivark »

quchen wrote: post #34
effie wrote: post #7

Earlier quoted context omitted.

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.

> 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 render…

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.

Re: Is light fundamentally a wave or a particle?

#64

Post by gpsx »

Most people with exposure to any physics learn some basics about electrons, light and quantum mechanics, but I think it is tough to put these different ideas together. The hard part about quantum mechanics for physicists is not the rules for quantum mechanics but how that manifests itself in the real world. This is how I have understand it.

I'll talk about electrons first. An electron is a point particle. The position of the electron is governed by a wave (wave function). The wave is not made out of "electron". It is made out of something else, kind of like "probability". The wave tells you where you find the electron. But when you do find the electron, it is always a point particle. You will never detect the electron smeared out over space. (As an aside, for those who think about field theory, I still see the electron as point in field theory, though some might see it differently.)

Light is a field/wave made of the electric field. And it has a wave function too. The wave function again is made of probability. But it is probability for different electric field configurations, rather than photon locations, so it might be harder to picture.

Why does a light behave like a particle at all? Though the light has spatial extent it does come in discrete chunks, specific quanta of energy. When light is absorbed it can only be absorbed in these discrete quanta. In most detectors (like photographic paper) the light is absorbed by an electron. Since the electron is a point particle, the photon absorption happens at a single point. This might make it look as if a "particle" of light hit the detector.

Re: Is light fundamentally a wave or a particle?

#65

Post by jfengel »

Betteridge's Law applies nicely here. The answer is "no".

Light is neither a wave nor a particle. It is a quantum thing whose formula can be simplified to either wavelike or particlelike under certain assumptions, but not in the general case. Experiments that force those assumptions will reliably turn up wavelike and particlelike behavior, while more general experiments reveal the harder case where the assumptions don't hold.

This really isn't a hard thing. This has been known for pretty much a century. People keep insisting that quantum mechanics is really just classical mechanics with an extra bag on the side, and that approach will always fail in some limit case.

When you accept that it's the reverse -- that classical mechanics is a special case of quantum mechanics -- you can work with QM in a very straightforward way. It's just not the straightforward way you're used to.

Re: Is light fundamentally a wave or a particle?

#67

Post by greysphere »

konschubert wrote: post #32
greysphere wrote: post #26

Earlier quoted context omitted.

"When it hits the walls it registers as a single point" is pretty vague and seems to me to be where you're kinda glossing over a lot of the not-just-a-wave complexity. Im fairly certain you could make an experiment where one photon went through some apparatus and two specks of light appeared at the photo detector (with new wavelengths of course). What mechanism in your wave-only theory describes this split? Why do th…

No, you can't do that with one photon.

So, 'something' must be able to turn a photon of one wavelength into a photon of another wavelength, otherwise we'd only have 1 wavelength of light in our universe.

Moreover, if we assume conservation of energy, changing a photon w/ wavelength A into B leaves over some energy C, that again, assuming some mechanism of changing energy into light, a new photon could be emitted.

So which assumption is wrong? (1) Light comes in many different wavelengths (2) Conservation of energy (3) Non-light energy can be changed into light energy (4) Light color follows the photon model for the relation between energy and wavelength.

(or just do a simple google search: https://arxiv.org/abs/2009.03039 where it's sounds like it's been observed directly)

Re: Is light fundamentally a wave or a particle?

#68

Post by jiggawatts »

witherk wrote: post #4

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…

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!

Re: Is light fundamentally a wave or a particle?

#69

Post by jiggawatts »

l33tman wrote: post #33
witherk wrote: post #4

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…

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 particu…

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.

Re: Is light fundamentally a wave or a particle?

#70

Post by jiggawatts »

These discussions always frustrate me, because for the ten thousandth time the same conversation plays out like a needle stuck on a record:

"So what is a particle?"

"Well, it acts like a wave but is not a wave."

"Aha..."

"It also acts like a particle but is not a particle."

"Okay, but then what is it?"

"I just told you! It's neither a particle, nor a wave, it's something else entirely!"

"I accept that it isn't either of those two things, but then describe the thing that it is!"

"I can't do that! You have to study a pile of textbooks this high to understand what it is!"

"Why? I understand relativity without having to have read a pile of textbooks! That's also an unintuitive physical concept with no classical analogy with which we are familiar with in every day life. Despite this it has intuitive visualisations to aid understanding! [1] Why can't you do something similar for quantum mechanics?"

"Wave-particle duality just has to be understood through the mathematics, there is no simpler way"

"Okay, okay, never mind. Now what about Spin?"

"Go away."

[1] https://www.youtube.com/watch?v=DYq774z4dws

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