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Why are some things darker when wet?

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Re: Why are some things darker when wet?

#151

Post by mrfox321 »

amluto wrote: post #145
aphextron wrote: post #18

This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new…

> The actual photons that hit an object are not the same ones that eventually enter your eyes. This part is philosophy, not physics. Photons are “indistinguishable particles”. You cannot tell two photons apart. This has a rigorous meaning: if you swap two photons, the state of the universe is unchanged. (This is in contrast to fermions — if you swap two fermions, the state of the universe has its phase shifted 180 de…

Electrons are indistinguishable as well.

Re: Why are some things darker when wet?

#152

Post by raldi »

amluto wrote: post #145
aphextron wrote: post #18

This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new…

> The actual photons that hit an object are not the same ones that eventually enter your eyes. This part is philosophy, not physics. Photons are “indistinguishable particles”. You cannot tell two photons apart. This has a rigorous meaning: if you swap two photons, the state of the universe is unchanged. (This is in contrast to fermions — if you swap two fermions, the state of the universe has its phase shifted 180 de…

> if you swap two fermions, the state of the universe has its phase shifted 180 degrees

Do you mean there are two kinds? Or if you swap any two?

Re: Why are some things darker when wet?

#153

Post by sjg007 »

amluto wrote: post #150
sjg007 wrote: post #149

Earlier quoted context omitted.

What surfaces reflect?

Metals and, generally, homogenous materials (dielectrics, conductors, etc). Light reflects off of a conductor or a dielectric due to a relatively straightforward solution to Maxwell’s equations. In contrast, anything phosphorescent most definitely absorbs and re-emits, sometimes hours later. Imagine glow-in-the-dark pigment.

Thanks, yes makes sense. But do metals actually reflect or do they just absorb and just release spontaneously?

Re: Why are some things darker when wet?

#154

Post by fennecfoxen »

hutzlibu wrote: post #82
reubenmorais wrote: post #77

Earlier quoted context omitted.

Light is not a wave. Photons are particles. Light as a wave is a convenient abstraction that explains a lot of light's behaviors, but not all of them. I recommend Feynman's QED lectures (or the book), where he explains this.

This debate is very old. Newton and Huygens fought long over it .. and eventually it was solved by: it is both or rather neither. It is a quant. https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality#...

A "quant" is someone who studies quantitative finance, typically found in investment fields. The word for this wave-packet with particle properties is "quantum". It is a singular noun; its plural is "quanta".

Re: Why are some things darker when wet?

#155

Post by anonytrary »

whatshisface wrote: post #49
aphextron wrote: post #18

This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new…

Having thought about it, I'm not sure if I agree. Light can reflect off of transparent crystals, which do not have the right electron energy levels for absorption (that's why they're transparent.) It's possible that the Feynman diagram of a reflection might involve the photon "going away and another one replacing it" (I don't know what the diagram looks like), but Feynman diagrams cannot be interpreted as sequences o…

> Light can reflect off of transparent crystals

If light is reflecting off of something, it's precisely because that thing isn't transparent[0].

> if the mirror is moving towards or away from the light, the reflection will be Doppler-shifted to a higher or lower frequency.

This does not refute OP's assertion. If the photon is absorbed and a "different"[1] one emitted, the emitted photon is still emitted from a reference frame that is moving with some velocity, therefore the emitted photon will still be Doppler-shifted... The photon is still emitted at the correct energy level. The photon appears to have higher energy in the stationary reference frame.

[0] Disregarding human-centric definitions that have to do with visible spectrum.

[1] Thinking of the photons as "different" doesn't actually mean much when it comes to quantum mechanics; they're indistinguishable bosons.

Re: Why are some things darker when wet?

#156

Post by Enginerrrd »

zamadatix wrote: post #110
tsbinz wrote: post #91

Earlier quoted context omitted.

This is the question that should be higher up in the thread. This whole discussion makes no sense. Photons carry no hidden identity.

A photon doesn't need to carry a UUID for people to conceptualize a difference between a photon going through free space and being absorbed and re emitted. The parent comment asks the right question, what is being meant by "the same" is required to know before being able to say the discussion makes no sense. Taking your first literal interpretation as the only way the discussion could have value is no more helpful th…

>The parent comment asks the right question, what is being meant by "the same" is required to know before being able to say the discussion makes no sense.

This is true, but I think the comment you are replying to is correctly identifying a fundamental conceptual misunderstanding of the comments preceding it. In the sense that they likely mean, it's not really a valid question. Sort of like the "are you still beating your wife?" question. Asking it is making incorrect assumptions.

Re: Why are some things darker when wet?

#157

Post by ddingus »

amluto wrote: post #150
sjg007 wrote: post #149

Earlier quoted context omitted.

What surfaces reflect?

Metals and, generally, homogenous materials (dielectrics, conductors, etc). Light reflects off of a conductor or a dielectric due to a relatively straightforward solution to Maxwell’s equations. In contrast, anything phosphorescent most definitely absorbs and re-emits, sometimes hours later. Imagine glow-in-the-dark pigment.

It was fun to think about glow in the dark pigments that way.

Re: Why are some things darker when wet?

#158

Post by ddingus »

thelittleone wrote: post #148
aphextron wrote: post #18

This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new…

If I recall a high school physics class correctly the colour of the photons that are emitted from the object (the colour we see) are the ones the object does not absorb. If so, is the object the colour we see or is it the colour it absorbs.

Color may not actually be a basic physical property.

Purple is an artifact of our minds, for example. Does not exist as a distinct wavelength of light.

The object absorbs, does not absorb, emits, whatever wavelengths of light. To us, those have colors, where they are in the visible spectrum.

Take us out of the equation, and?

Re: Why are some things darker when wet?

#159

Post by amluto »

raldi wrote: post #152
amluto wrote: post #145

Earlier quoted context omitted.

> The actual photons that hit an object are not the same ones that eventually enter your eyes. This part is philosophy, not physics. Photons are “indistinguishable particles”. You cannot tell two photons apart. This has a rigorous meaning: if you swap two photons, the state of the universe is unchanged. (This is in contrast to fermions — if you swap two fermions, the state of the universe has its phase shifted 180 de…

> if you swap two fermions, the state of the universe has its phase shifted 180 degrees Do you mean there are two kinds? Or if you swap any two?

Swap any two.

This is why two fermions can’t be in the same state: if you swapped them, you’d be back where you started, but that somehow also had to phase shift 180 degrees, and the only complex amplitude that is the same as its own negation is zero.

Re: Why are some things darker when wet?

#160

Post by chemeng »

viklove wrote: post #142
chemeng wrote: post #59

Earlier quoted context omitted.

They are not the same photons. They are emitted by the electron changing energy states after excitation of the incoming photon. But your intuition that there is some quantum weirdness is correct. Quantum Electrodynamics is the theory you're looking for. It unifies quantum mechanics and special relatively in the context of light/matter interaction. Given you seem to be familiar with Feynman. You're in for a treat! His…

But that's just not true. The electron will only get excited if the incoming photon has a very specific wavelength, so most of the light that hit your house's walls, for example, is reflecting. The photoelectric effect is real, but it only applies in specific circumstances...

There will be a transfer of energy to the system if there is an interaction, but will only absorb the incident photon at specific energies. In the case of reflection in this classical view, it simply re-emits the energy back out as there are no valid energy transitions. The reality is far more complex and this thread is mixing multiple theories together, wave and particle views and simultaneously comparing single atom mechanics with those of lattices/solids. This is all causing a lot of confusion that's hard to address in comments.
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