Photons

[Chesterton]

From what I understand, the science isn't clear on whether photons are particles or waves, because they seem to have characteristics of both.

When light lands on a physical object, some of it's reflected and some of it's absorbed (and in the case of transparent material like glass or water, most of it just passes through).

My question is: what's happening on the level of the individual photon? Are some of them (if they are particles) bouncing off things, and some of them absorbed, or are a percentage of them (if they are waves) doing the same? Can we determine the percentage?

If I can see a rock in my backyard, it's because the photons are bouncing off of it and striking my eyeballs. But on a hot summer day, I can also touch the the rock and feel warmth because of those same photons.

 

[essentialsaltes]

From what I understand, the science isn't clear on whether photons are particles or waves, because they seem to have characteristics of both.

Science is very clear on what a photon is. Unfortunately, it is neither what we commonly think of as a particle, nor a wave. Quanta behave like quantum mechanical objects.

For short, we talk about individual photons as particles. And we have a branch of physics called particle physics. But to the physicist these 'particles' have properties that are both wave-like and particle-like.

When light lands on a physical object, some of it's reflected and some of it's absorbed (and in the case of transparent material like glass or water, most of it just passes through).

My question is: what's happening on the level of the individual photon? Are some of them (if they are particles) bouncing off things, and some of them absorbed,

That is correct.

If I can see a rock in my backyard, it's because the photons are bouncing off of it and striking my eyeballs. But on a hot summer day, I can also touch the the rock and feel warmth because of those same photons.

Yes, some of the photons (from the sun) have been absorbed by the rock, making it warmer. Others, as you say, were reflected in a direction to get into your eyes, where they are absorbed by your retina (which not only warms your retina, but contributes to the signal that gets sent to the vision center of your brain.)

 

[Chesterton]

Let me simplify the question: 100 photons strike an object. How many are reflected and how many are absorbed?

 

[essentialsaltes]

Let me simplify the question: 100 photons strike an object. How many are reflected and how many are absorbed?

Depends on the material and the wavelength. But whatever the answer is, it will be a whole number.

 

[Mystman]

Depends on the material and the wavelength. But whatever the answer is, it will be a whole number.

And may my various quantum mechanics teachers forgive me if I get this wrong: if you repeat the experiment with the exact same material and wave length, you are likely to get a different number each time.

 

[Elendur]

Depends on the material and the wavelength. But whatever the answer is, it will be a whole number.

Doesn't the angle matter as well?

And may my various quantum mechanics teachers forgive me if I get this wrong: if you repeat the experiment with the exact same material and wave length, you are likely to get a different number each time.

Isn't that what you'd expect when dealing with chance?

 

[florida2]

The idea of 'wave' and 'particle' are just nice, neat concepts which we are able to comprehend. The real nature of quantum mechanical objects is somewhat more ephemeral and difficult for us to get out brains around.

Subatomic particles such as electrons, protons and neutrons have been made to behave as waves - and even atoms and molecules. In fact, everything has a wavelength though things that we can see will have a wavelength many times smaller than a proton so seeing any wave-like behaviour is unlikely!

 

[essentialsaltes]

And may my various quantum mechanics teachers forgive me if I get this wrong: if you repeat the experiment with the exact same material and wave length, you are likely to get a different number each time.

Sure.

Doesn't the angle matter as well?

Sure, and no doubt the temperature.

 

[Chesterton]

And may my various quantum mechanics teachers forgive me if I get this wrong: if you repeat the experiment with the exact same material and wave length, you are likely to get a different number each time.

Why is that?

Sure, and no doubt the temperature.

But photons themselves cause temperature, don't they?

 

[essentialsaltes]

"you are likely to get a different number each time"
Why is that?

At the risk of giving too much information, this would probably also be true in most ordinary classical 'experiments' as well, unless they were impossibly well controlled. Suppose we were firing 100 tennis balls at a surface covered in sticky velcro. Some tennis balls will stick (be absorbed), while others will bounce off. If we did the experiment twice, probably different numbers of tennis balls would stick, because there are so many variables: the spin on the balls, the exact shape of the fuzz on the tennis balls, and the little hooks on the velcro, breezes in the air....

OK, now here's where it possibly gets weird. If we could somehow make sure that each tennis ball was exactly the same, and travelled in exactly the same path, and met the same patch of velcro. If the speed and spin of the ball flying through the air was the same to the nth decimal place...

well, classical mechanics is deterministic. The past state of a system uniquely determines its future state. If the variables are all the same, the same thing will happen. If we could do this impossibly well-controlled experiment, the results would either be 100 stuck balls or 100 bouncing balls.

Quantum mechanics is nondeterministic. There is an inherent element of chance in quantum events. If we could again set things up so that each of the 100 photons was the same, and the exact spot it hits was also the same, then there would still be uncertainty in the outcome. The quantum interaction of the photon with the surface might be such that 40% of photons will get absorbed, and 60% will bounce. Each photon 'rolls the dice' and one of the two outcomes occurs. Since this is now a statistical event, running the experiment twice will have different results. Just like if you flip 100 coins, you are not likely to get exactly 50 heads and 50 tails, and if you do it again, you probably won't get the same results as the first 100.

But photons themselves cause temperature, don't they?

Photons are not the only cause of heating. But yes, absorbing photons would lead to an increase in temperature. (Although I should note that the temperature increase from a single photon would be on its own virtually undetectable.) This is unrelated to the point I was making, which was just that temperature is another variable that would probably affect the 'odds'. At 100 degrees maybe 40% of photons are absorbed, but at 300 degrees, 30% are absorbed.

 

[Subduction Zone]

Wouldn't if be possible for more quanta to be radiated from the object eventually than struck it? For example is it possible for a photon to cause an electron in an atom to jump up two quantum states but then radiate out that energy in two separate lower level energy quantum jumps?

 

[essentialsaltes]

Wouldn't if be possible for more quanta to be radiated from the object eventually than struck it? For example is it possible for a photon to cause an electron in an atom to jump up two quantum states but then radiate out that energy in two separate lower level energy quantum jumps?

This is true.

But when the first photon "cause an electron in an atom to jump up two quantum states" that would be the photon being absorbed. So that's how it would be counted in our thought experiment.

The two lower energy photons that emerged would have a different wavelength than the initial photon, so we would know that these weren't reflected photons (which would have the same wavelength as the initial photon).

(and I guess for completeness, another possibility beyond absorption and reflection would be the Raman effect. Or if we happened to be using X-ray photons, the Compton effect.)

 

[Chesterton]

At the risk of giving too much information,...

What, is there some kind of secret I'm not allowed in on?

...this would probably also be true in most ordinary classical 'experiments' as well, unless they were impossibly well controlled. Suppose we were firing 100 tennis balls at a surface covered in sticky velcro. Some tennis balls will stick (be absorbed), while others will bounce off. If we did the experiment twice, probably different numbers of tennis balls would stick, because there are so many variables: the spin on the balls, the exact shape of the fuzz on the tennis balls, and the little hooks on the velcro, breezes in the air....

OK, now here's where it possibly gets weird. If we could somehow make sure that each tennis ball was exactly the same, and travelled in exactly the same path, and met the same patch of velcro. If the speed and spin of the ball flying through the air was the same to the nth decimal place...

well, classical mechanics is deterministic. The past state of a system uniquely determines its future state. If the variables are all the same, the same thing will happen. If we could do this impossibly well-controlled experiment, the results would either be 100 stuck balls or 100 bouncing balls.

Quantum mechanics is nondeterministic. There is an inherent element of chance in quantum events. If we could again set things up so that each of the 100 photons was the same, and the exact spot it hits was also the same, then there would still be uncertainty in the outcome. The quantum interaction of the photon with the surface might be such that 40% of photons will get absorbed, and 60% will bounce. Each photon 'rolls the dice' and one of the two outcomes occurs. Since this is now a statistical event, running the experiment twice will have different results. Just like if you flip 100 coins, you are not likely to get exactly 50 heads and 50 tails, and if you do it again, you probably won't get the same results as the first 100.

Are you saying there is such a thing as randomness? And is the universe fundamentally lawless?

 

[essentialsaltes]

Are you saying there is such a thing as randomness? And is the universe fundamentally lawless?

The philosophers will argue about what quantum mechanics means, but to this humble physicist, it appears that yes, quantum events are inherently random. If you have two identical uranium atoms, it is impossible to predict which of them will undergo radioactive decay first, or when the first one will decay.

This does not mean that the universe is fundamentally lawless, just that some of the laws are probabilistic. The casinos in Vegas can make some pretty predictable profits using the laws of probability. And even for quantum mechanics, if you have 2 quadrillion identical uranium atoms, we can predict pretty precisely how long you'd have to wait until you only had 1 quadrillion uranium atoms. That would be the half-life of the uranium isotope.

The inherent randomness of QM does not mean that 'anything can happen'.

 

[Subduction Zone]

This is true.

But when the first photon "cause an electron in an atom to jump up two quantum states" that would be the photon being absorbed. So that's how it would be counted in our thought experiment.

The two lower energy photons that emerged would have a different wavelength than the initial photon, so we would know that these weren't reflected photons (which would have the same wavelength as the initial photon).

(and I guess for completeness, another possibility beyond absorption and reflection would be the Raman effect. Or if we happened to be using X-ray photons, the Compton effect.)

Thank you, perhaps I misread one of your posts but it seemed to imply that only as many photons that were absorbed could be emitted, but then you were using a analogy and they always have limitations.

 

[Chesterton]

The philosophers will argue about what quantum mechanics means, but to this humble physicist, it appears that yes, quantum events are inherently random. If you have two identical uranium atoms, it is impossible to predict which of them will undergo radioactive decay first, or when the first one will decay.

This does not mean that the universe is fundamentally lawless, just that some of the laws are probabilistic. The casinos in Vegas can make some pretty predictable profits using the laws of probability. And even for quantum mechanics, if you have 2 quadrillion identical uranium atoms, we can predict pretty precisely how long you'd have to wait until you only had 1 quadrillion uranium atoms. That would be the half-life of the uranium isotope.

The inherent randomness of QM does not mean that 'anything can happen'.

I don't mean this to sound insulting, but isn't there a bit of human arrogance involved here? Because you physicist guys can't predict something, you assume it's random?

And a joke: What did one casino owner say to the other? "What I love about this job is the exciting unpredictability. Some days you win, some days you win a whole lot."

 

[Elendur]

I don't mean this to sound insulting, but isn't there a bit of human arrogance involved here? Because you physicist guys can't predict something, you assume it's random?

And a joke: What did one casino owner say to the other? "What I love about this job is the exciting unpredictability. Some days you win, some days you win a whole lot."

The thing about randomness is that there's different kinds of random.

There's entire fields of mathematics devoted to different types of randomness.
And when we go deeper... *shudder*

For examples, look here:
List of probability distributions - Wikipedia, the free encyclopedia

A very usual example is if you flip a coin, that's randomness, but you can predict it quite well using simple logic.

 

[Chesterton]

A very usual example is if you flip a coin, that's randomness, but you can predict it quite well using simple logic.

If you flip a coin, or toss an entire deck of 52 cards in the air, physical laws will determine the outcome. I don't see how there can be such a thing as chance or randomness.

 

[Elendur]

If you flip a coin, or toss an entire deck of 52 cards in the air, physical laws will determine the outcome. I don't see how there can be such a thing as chance or randomness.

There's at least the concept.

Whether something is truly random or not is irrelevant, we can describe events accurately with the mathematical properties that randomness has.

There's something important with remembering that when dealing with the natural world, we're approximating and filtering like crazy.

If you're thinking that the gravity equation of f=g*m1*m2/r^2 (or something like that) is proper, you're way of, it's a good approximation (and some would debate the "good"). No more.

 

[Chesterton]

There's at least the concept.

There's lots of concepts which may or may not be true.

Whether something is truly random or not is irrelevant, we can describe events accurately with the mathematical properties that randomness has.

Describing and predicting are two different things.

There's something important with remembering that when dealing with the natural world, we're approximating and filtering like crazy.

Yes. As a Christian, I get accused of approximation and filtration a lot. So we have something in common.