I understand enough physics to get that light travels in the form of photons, in a stream of tiny bundles of light which hit surfaces and either bounce off them or get absorbed.
So, say they bounce: what happens to them? Is the air around us filled with dispersed photons forever bouncing around? Do they disintegrate after they get bounced?
Thanks in advance.
All photons get absorbed eventually. If they didn’t, your surroundings would appear to constantly get brighter and brighter.
Since Q.E.D. answered the basic question, I just thought I’d point out that photons don’t “bounce”. They can be absorbed and then re-emitted, but they don’t bounce in the classical sense of the word. Also, photons are both particles and waves, not just particles.
Photons do not decay, they exist until they interact with something else.
Are you sure about this? The latest cosmology says the universe is open and its expansion is accelerating, so would all photons necessarily interact with something eventually? I’m thinking there may be some photons which “miss” the rest of the universe.
Infinity is a long time. The probility of *anything **happening during that timeframe is always 100%. 
*anything which is physically possible.
Okay, so let’s say Petunia the Photon (a mysterious yet alluring wave-particle) is emitted by the sun, has a fairly uneventful eight-minute journey to the third rock out, and is ultimately absorbed by this black table here.
Is it accurate to name Petunia? I had understood that photons were “quantum” - discrete - and that they remain so over time.
Is there some way of measuring whether Petunia or one of her posse is “present” after the photon beam has been dispersed / absorbed? Or does Petunia entirely cease being Petunia? Does she meaningfully exist as a discrete unit, when separated from all her photon friends?
What happens when they interact with something else?
I’m also wondering what (if anything) the uncertainty principle has to do with this.
I’m not certain that there’s some way of knowing whether a photon is present, or not. Can we say what happens to the photons with respect to the table only in terms of probability?
So, where is Petunia? Is the question even meaningful? Would she be in the table, or in the surrounding air, or halfway to Mercury? Or all three (plus infinite alternatives) with varying probability? (Am I completely off base here?)
Don’t ask me why I’m thinking about this. For some reason Christmas always causes me and my husband to get in arguments about physics. Last year around this time y’all kindly helped us figure out the physics of swings at the park.
Little Petunia Photon has two important quantities: energy (the potential to do work) and momentum (the direction it’s going, and how much “kick” it has). When Blackie Carbon Atom in the table absorbs Petunia, that’s it for her. Blackie now has her energy and momentum.
Of course, Blackie may not keep that energy and momentum forever. Thus maybe Penny Photon is born. Penny may have the same energy and momentum that Petunia had, but chances they’re different. So Penny reallly is a different photon.
Ah, I love parables.
Interesting!
So Petunia is nothing but the current instantiation of some energy and momentum, the vehicle by which it arrives from the sun?
What if there’s no carbon in my tabletop? Can Ulf the Undetermined Element, or whoever happens to be there, do the same job?
And if it’s some other atom that has some other colour, which absorbs some and scatters others? Can we know what happens to Petunia in particular?
I think the answers to your questions would take on the form of a book with around 20 chapters.
According to uncertainty you cannot know where Petunia is without influencing where she is.
You should also bear in mind that Petunia doesn’t have “an uneventful 8 minutes” coming from the sun, she has no time at all. Travelling at the speed of light means from her point of view she does not travel in time.
So Petunia is at all possible (or probable) points (including a divertion via the far side of the Universe) between the sun and your table, all the time.
Beyond this, you’re going to need more than just the one thread…
Awesome. I think that’s what I was getting at with this
WOW! I love this shit. It’s so fun to boggle the mind!
That’s a fairly good description of a photon.
Any atom has a chance to absorb any photon, but the exact probability depends on the atom (and it’s electrons’ current states) and the frequency (color) of the photon. If the photon is particularly big (radio-frequency, for example) a large collection of atoms may be required to absorb it (think tv antenna).
It’s difficult to determine the life of an actual photon. As, Futile mentions, we can’t determine the details about Petunia without changing her.
And naming the carbon atom Blackie may have slightly mislead you: it’s not too meaningful to assign a color to a single atom.
Actually, I’m not sure it’s that simple. An object that absorbs a photon will eventually either have to emit energy or will get so hot that it radiates. This is at the heart of
Olber’s Paradox
Olber’s Paradox can also be resolved (besides a universe with finite physical or temporal extent) if space-time is not flat. Distortions like gravitational lensing could make some parts of the sky darker/brighter than other parts.
Another possibility is that visible light could be shifted into other parts of the spectrum.
Hmmm, I thought that photons always move at the speed of light, and the apparent slower motions are caused by their interactions with other particles, in example two photons, Phil and Fred travel togheter through vacum, Phil enters a glass block and is absorbed, emited, absorbed, emited a gazillion times. When Phil´s helluva-grand-chilren comes out the other side of the block Fred, who didn´t have all those “bounces” is taking the lead…
Right? :dubious:
You are absolutely right. And so am I.
A photon’s energy is E = hf, where h is Planck’s constant and f is the photon’s frequency. And a photon’s momentum is p = hf/c, where c is Einstein’s constant (i.e., the speed of light in a vaccum). Frequency is the important quantity here.
You’re probably getting confused with the formulas for massive objects, which depend on speed. Kinetic energy is K = 1/2 mv[sup]2[/sup] and momentum is p = mv, where m is the mass and v the speed of the object.
Yay! Post 400!
Here’s something I’ve always wondered: If atoms reflect light by absorbing the photons and transmitting other photons, how is it that mirrors reflect all photons (or nearly all photons) at the complimentary angle of the original path and the plane of the object? When you get down to that level, the atoms are spherical so it seems magical that the atom transmits it in exactly the right direction as if it knows where the atoms around it are. The refraction of photons doesn’t make sense to me either. How do you explain mirror reflection and refraction on the atomic scale?
Ehh… I think I´m missing some part of the explanation because I don´t get it.
I just want to add my standard (on this board) semi-alternative view that quantum field theory is the best way to look at this. That is, there aren’t “photons”, but rather a single “photon field”. Photons are just basic excitations of this field. Thinking of them as little bundles of energy really misses their essential nature, which explains rather elegantly how this field interacts with other fields.
I don’t pretend to know the full answer, but just consider this:
The atoms on the surface of the mirror do not exists on their own, but there are chemical bounds between them, which means that their electron shells are distorted and overlap. So the electron cloud that interacts with the photon is not spherical after all.
BTW, the electron clouds are not spherical even for single atoms (except the most simple ones, hydrogen and helium), but they form lobes.
The same way as it’s explained on the classical scale: the principle of extremal action.