question about light and mass

[MrDudeMan]

ok, i have a question about light being a wave and a particle...how can you tell which it is at the moment? and i also dont understand how it has no mass because you get warm from light hitting you. doesnt it have to have a little mass to transfer heat? i know waves can heat stuff up, like a microwave, but that would mean ALL light is a wave, but i know its not (or, i know its not because thats what ive been told).


if it can be in either state, what determines that? when it is emitted from the sun...what state is it in?

 

[ZeroNine8]

Without knowing if light has been definitely determined to be a particle or a wave, I think this discussion stems from the fact that sometimes light is modeled and behaves more like a wave and sometimes it is modeled and behaves more like a particle. I think right now that we don't understand enough about the nature of such things to really classify it, so we use the models we have when they apply. Whatever light actually is, it exhibits both wave and particle properties, though this does not mean it is purely a wave or purely a particle.

 

[sash1]

a perfect example to explain this is the double slit experiment. The experiment shows how light is both a paricle and a wave.

When you emit light, it starts out as a particle, when it travels it is a wave, and when it hits the light plate at the end of the experiment, it is again a particle.

I wish I could find a picture of it, but here it is:

.....|.......|....)
.....|.......|..)
.....|.....)...)
.....|...)...|..)
->....).....|....))
.....|...)...|..)
.....|.....)...)
.....|.......|..)
.....|.......|....)

-> = light emitter
) = light waves
)) = interference
| = wall

at the end there is a plate where the particles of light hit. There is a stronger concentration in the middle, where there is interference, but the outskirts have particles hitting as well. If light were a particle, it would only hit the center. However, this experiment seems to prove the duality of light.

As well, you are correct. Light does have mass. It has no rest mass, though light can never be at rest (though I should refrain from saying this, as scientists have been able to do it, though for all intents and purposes, light is constantly at c).

Hope this gives you a starting point. If I find a good picture of the double-slit experiment I'll share it.

`K

 

[Geniere]

If you do an experiment designed to test the particulate property of light, your experiment will prove it is a particle. If you do an experiment designed to test the wave property of light, your experiment will prove it is a wave. The double slit experiment was designed to test the wavelike properties of light and found light propagates as wave. Conversely the photoelectric effect experiment, designed to test the particulate properties, found light (photon) to be a particle. When light leaves the sun it is in the same state that all the photons in the universe are, that being, it will exhibit either property depending on the way it is tested.

Physicists no longer speak of rest mass. When a particle is described as having X mass, that always refers to its ?rest? mass. In Einstein?s famous equation, E=MC**2, the M refers to and only refers to the objects rest mass. If one wants to calculate the relativistic energy of an electron one uses a similar but different equation. It is only at great speeds where large differences in results will be obtained.

 

[PowerEngineer]

I think the way to view light's seemingly dual wave/particle nature is as a shortcoming in our ability to grasp its true nature.

We observe the behavior of the universe around us and we try to develop explanations (i.e. theories) that describe what we're seeing. These theories almost always try to describe something strange and otherwise beyond our mental grasp in terms of other physical phenomena we're more comfortable and familiar with. We have theories about how light works that are based on our comfortable ideas about particles and about waves (both physical phenomina we experience pretty directly in the world around us). These theories each seem to describe some aspects of light's nature, but neither describes light completely.

It's a little bit like this. Let's say "light" is really equal to 92. Now the particle treatment of light says to us that "light" is greater than 73, and the wave treatment of light tells us that "light" is less than 132. Now that's useful information; we now that 73 < "light" < 132. That's truth as far as we can take it -- but it doesn't really get to truth that "light" is 92.

Of course, this explanation itself is an example of trying to describe an unfamiliar concept in terms of what might be more familiar!

 

[MrDudeMan]

ok but how do we know how to harness it then? example, how does a flashlight work? if it was a wave, would it still reflect off of the parabolic shape of the inside mirror?

i guess im just having a really hard time grasping the concept, but as it seems, so is everyone else. your posts were extremely detailed and everyone makes good points, but i still just cant seem to begin to understand this. how can it be 2 different things seemingly whenever the hell it wants? what determines its current phase? is there anyway to make it ONLY one? is there anyway to tell EXACTLY what it is at that exact moment?

something you guys didnt touch on is its ability to heat something up. how does light heat up food (heat lamps) or burn your skin? if it is massless, that should be impossible, right? but it has mass apparently, so what gives? another thing is, why is it affected by black holes if it is massless, and on the contrary, why is it able to escape the pull of a black hole if it does have mass? if it was in wave form, how would it even get sucked into a black hole? how does a wave experience the pull of gravity? so many things that are hard to comprehend

 

[ZeroNine8]

i guess im just having a really hard time grasping the concept, but as it seems, so is everyone else. your posts were extremely detailed and everyone makes good points, but i still just cant seem to begin to understand this. how can it be 2 different things seemingly whenever the hell it wants? what determines its current phase? is there anyway to make it ONLY one? is there anyway to tell EXACTLY what it is at that exact moment?

I think that one of the better explanations is that light is neither a particle nor a wave, rather something else that only exhibits some properties of particles and some properties of waves. Whatever light actually is, it manifests both wavelike and particle like properties, however this does not mean that it IS a particle or wave, it merely 'looks' like one with our current detection and measurement experiments. Here is sort of an example of this idea: imagine you know nothing about what a television screen is. If the image on the screen is a fireplace, you might think the screen actually is a fireplace, or if it has fish on the screen, you might think it is an aquarium. The point is that the television screen is something else entirely that appears to be something which is familiar, either an aquarium or fireplace in this case. Until you become familiar with how a television works, you cannot explain what you see other than saying that the same thing appears to be two completely different things.

ok but how do we know how to harness it then? example, how does a flashlight work? if it was a wave, would it still reflect off of the parabolic shape of the inside mirror?

Knowledge about how something works is not necessary to harness it, how long was fire in use before people figured out that it was a release of energy by breaking chemical bonds through combustion? We don't know how gravity works exactly either, only that it does work and appears to be an attractive force between two masses, but the why and how are still a mystery. Knowledge about how something works has absolutely no bearing on the fact that it works. Fire, perhaps the strongest force in the devlopment of human civilization, works without regard for our knowledge of it. Additionally, we are able to control fire by adding fuel, dousing it with water, starving it of oxygen, however none of these things were found based on a scientific explanation of the chemical reaction being inhibited. People found that they worked, so they just said 'hey, if you add fuel, the fire burns more' or 'water puts fire out', and that worked and continues to work with or without explanation of why.

 

[sao123]

as simple as i can put it...

We model light as a wave when it is traveling through any void meduim...
IE observing (wavelength, frequency, amplitude...)

We model light as a particle, the moment it interacts with any other particle or a non-void medium- Atom, electron, etc...
IE observing (reflection, refraction, absorption, diffussion)

 

[silverpig]

I'd find it surprising to discover there are any perfect particles or perfect waves at all. People exhibit wave-particle duality as well. When you jump through the air, you have a wavelength (albeit extremely extremely small). People aren't perfect particles, and I doubt there actually are perfect particles.

 

[MrDudeMan]

Originally posted by: silverpig
I'd find it surprising to discover there are any perfect particles or perfect waves at all. People exhibit wave-particle duality as well. When you jump through the air, you have a wavelength (albeit extremely extremely small). People aren't perfect particles, and I doubt there actually are perfect particles.

i think that is pushing it a little...we are composed of particles, and only particles. i dont think how you observe something explains what it is.

 

[Hayabusa Rider]

Originally posted by: Kauru
a perfect example to explain this is the double slit experiment. The experiment shows how light is both a paricle and a wave.

When you emit light, it starts out as a particle, when it travels it is a wave, and when it hits the light plate at the end of the experiment, it is again a particle.

I wish I could find a picture of it, but here it is:

.....|.......|....)
.....|.......|..)
.....|.....)...)
.....|...)...|..)
->....).....|....))
.....|...)...|..)
.....|.....)...)
.....|.......|..)
.....|.......|....)

-> = light emitter
) = light waves
)) = interference
| = wall

at the end there is a plate where the particles of light hit. There is a stronger concentration in the middle, where there is interference, but the outskirts have particles hitting as well. If light were a particle, it would only hit the center. However, this experiment seems to prove the duality of light.

As well, you are correct. Light does have mass. It has no rest mass, though light can never be at rest (though I should refrain from saying this, as scientists have been able to do it, though for all intents and purposes, light is constantly at c).

Hope this gives you a starting point. If I find a good picture of the double-slit experiment I'll share it.

`K

ummm, AFIK no experiment has detected a mass in a photon, and if one is detected, many theories would have a problem. Link for the upper mass of a photon.

BTW, particles have duality too. Electrons can display the same behaviors as photons.

 

[PowerEngineer]

Originally posted by: MrDudeMan

Originally posted by: silverpig
I'd find it surprising to discover there are any perfect particles or perfect waves at all. People exhibit wave-particle duality as well. When you jump through the air, you have a wavelength (albeit extremely extremely small). People aren't perfect particles, and I doubt there actually are perfect particles.

i think that is pushing it a little...we are composed of particles, and only particles. i dont think how you observe something explains what it is.

I know it seems strange, but it is true that matter has both particle and wave natures too.

The particle nature of matter dominates when dealing with the masses we deal with in our daily lives (and therefore seems more comfortable and logical), however the wave nature of matter begins to take over at the atomic level. There's really no "particle" explanation for the orbits/energy levels of electrons, and seemingly no way to know exactly where an electron really is in its "orbit". Quantum physics/mechanics is essentially the study of the wave nature of matter (and I often found its concepts disconcerting and illogical).

One interesting conjecture I've heard is that "consciousness" might be linked to the quantum waves established by the atomic structures in the brain, opening up the possibility that it's more than just chemical. The quantum wave might be our "soul", and that our "free will" might somehow come out of Hiesenberg's uncertainty principle. A thought provoking idea!

 

[MrDudeMan]

Originally posted by: PowerEngineer

Originally posted by: MrDudeMan

Originally posted by: silverpig
I'd find it surprising to discover there are any perfect particles or perfect waves at all. People exhibit wave-particle duality as well. When you jump through the air, you have a wavelength (albeit extremely extremely small). People aren't perfect particles, and I doubt there actually are perfect particles.

i think that is pushing it a little...we are composed of particles, and only particles. i dont think how you observe something explains what it is.

I know it seems strange, but it is true that matter has both particle and wave natures too.

The particle nature of matter dominates when dealing with the masses we deal with in our daily lives (and therefore seems more comfortable and logical), however the wave nature of matter begins to take over at the atomic level. There's really no "particle" explanation for the orbits/energy levels of electrons, and seemingly no way to know exactly where an electron really is in its "orbit". Quantum physics/mechanics is essentially the study of the wave nature of matter (and I often found its concepts disconcerting and illogical).

One interesting conjecture I've heard is that "consciousness" might be linked to the quantum waves established by the atomic structures in the brain, opening up the possibility that it's more than just chemical. The quantum wave might be our "soul", and that our "free will" might somehow come out of Hiesenberg's uncertainty principle. A thought provoking idea!

woah...over stimulation...can not compute...

i love reading your HT posts dude...i am the kind of person that loves to think about stuff which has no answer and you are really good at providing me with that kind of material


i still dont get how particles can act like waves, but i dont think im going to even touch on that for a while

 

[Fencer128]

i still dont get how particles can act like waves, but i dont think im going to even touch on that for a while

The problem you have grasping this is compounded by your initial views. When analysed objectively light appears to act as a wave or a particle depending on the situation, and sometimes the interpretation applied to the data from the experiment.

This is not to say it IS a wave or a particle, just that with our limited conceptual abilities we envisage it as such. It may be some other physical variant we do not understand and cannot interpret as a part of the world we understand at this moment.

Cheers,

Andy

 

[uart]

Originally posted by: WinstonSmith

Originally posted by: Kauru

... As well, you are correct. Light does have mass. It has no rest mass, though light can never be at rest ....

ummm, AFIK no experiment has detected a mass in a photon, and if one is detected, many theories would have a problem. Link for the upper mass of a photon.

BTW, particles have duality too. Electrons can display the same behaviors as photons.

I thought that light had momentum, rho = h / lambda, is that correct ? (BTW rho = momentum, h = Plancks constant and lambda = wavelength). So if photons have momentum and speed then why not mass, or at least relativistic mass.

 

[ZeroNine8]

You can't have momentum without mass, so if a photon has momentum (no clue about the validity of that claim) then I would think that it would also have to have an associated mass. Units of momentum are kg*m/s, or mass*velocity.

 

[Matthias99]

Yeah, that confused me greatly (and still does). Light has no *rest* mass, but it's also impossible to have a photon at rest.

When a photon is moving (at c), it has momentum. I'm not certain if it technically has "mass" or not (not, I'm thinking, although they are affected by gravity... ow. My head hurts now.)

Basically, if a photon runs into you, or you emit a photon, some amount of kinetic energy gets transferred back and forth. It's very, *very* small, but if you have a lot of photons (or a lot of time), it makes a difference. An article on lightsails might have more information on this.

 

[MrDudeMan]

Originally posted by: Matthias99
Yeah, that confused me greatly (and still does). Light has no *rest* mass, but it's also impossible to have a photon at rest.

When a photon is moving (at c), it has momentum. I'm not certain if it technically has "mass" or not (not, I'm thinking, although they are affected by gravity... ow. My head hurts now.)

Basically, if a photon runs into you, or you emit a photon, some amount of kinetic energy gets transferred back and forth. It's very, *very* small, but if you have a lot of photons (or a lot of time), it makes a difference. An article on lightsails might have more information on this.

yeah i was thinking about those little shiny things in sealed boxes that move when you shine a flashlight on them or put them in the window. how does that work if a photon has no mass?

 

[rjain]

Originally posted by: ZeroNine8
You can't have momentum without mass, so if a photon has momentum (no clue about the validity of that claim) then I would think that it would also have to have an associated mass. Units of momentum are kg*m/s, or mass*velocity.

Photons don't exist in the physics you are talking about, so talking about the mass or momenutm of a photon is impossible.

 

[rjain]

All matter (which includes energy) acts as both a wave and a particle at all times. How localized the wave is determines how much like a particle it would act, which is influenced by its wavelength (which is influenced by its momentum).

A photon has no rest mass, meaning that all its mass comes from the energy of its wave motion, planck's constant times the frequency. Light as both mass (energy) and momentum. You can build a light sail, which catches the momentum and energy of light in the same way that wind sails capture the momentum and energy of air. I believe that the thin golden sails in Star Wars are supposed to be light sails.

 

[ZeroNine8]

So the fact that mass and energy are interchangeable lets you have momentum with no 'mass' in the classical sense. I'll be really interested to see the unified theory of everything if it ever gets discovered .

 

[Jeff7]

Light's dual nature makes it seem almost as though a tiny particle is orbiting some unseen component, and that this very rapid movement is what creates the wave form.
Or maybe I'm just talking out of my

.




(The rose of course refers to...well, it don't smell like roses, put it that way. )

 

[rjain]

Jeff7: It's good to hear you have a rose growing out of your body, which can be used as a speech organ.

The wave nature of particles is from the fact that the amplitude of the particle is a field. It has different values throughout space and interferes with itself and other fields. The particle nature is due to the fact that the wave is quantized and the spreading of the wavefunction seems to stop at some point and "decohere".

 

[silverpig]

Originally posted by: ZeroNine8
You can't have momentum without mass, so if a photon has momentum (no clue about the validity of that claim) then I would think that it would also have to have an associated mass. Units of momentum are kg*m/s, or mass*velocity.

Ah but they do have momentum. In fact a russian team is launching a solar sail prototype in the very near future. It will use the transfer of momentum from the sun's photons to push itself into a higher orbit around earth. And E = mc^2 remember...

 

[rjain]

E=mc^2 has nothing directly to do with the momentum of a photon.

It's p = hv, where v is actually the greek letter nu, representing the frequency. h is planck's constant.