I’ve always heard that it is information that cannot travel faster than light, and I’ve taken this (maybe incorrectly) to mean that photons carry information. But what information? Something about the last subatomic properties of the particle that it bounced off of?
2. How do photons originate?
Are they caused by particle collisions or something? Do they come out of nothing? If so, by what mechanism?
3. How is a photon destroyed?
Is there an anti-photon by which the two will annihilate? Will an electron beat a photon, like rock on scissors? Do black holes destroy them?
I’m not even close to a physicist (yet) but I think that you’re trying to apply conventional events of the macroscale to light, which stradles the boundary between particle and wave to begin with. Maybe some of your questions do have answers I don’t know, but I’d suspect that these things don’t apply at the photon level they way they do in the “universe” that we observe every day.
I can accept that if that’s the case. But if it is, I’d appreciate a really clear explanation as to why such concepts as information, origin, and destruction do not apply at the quantum level.
None I think. Photons of specific wavelengths and/or energies can be measured and scientists can deduce what the photon came from (i.e. certain substances always emit light of a certain color…in this way scientists can determine what a glowing object is made of). Likewise, a certain pattern of photons can carry information as through a fiber-optic cable. However, I don’t think you can say a photon itself carries information. 2. How do photons originate?
Just above I mentioned that a photon could tell someone what it came from by measuring its energy/wavelength (remember light can be considered both a wave and a particle simultaneously). Atoms have electrons orbiting them. Different types of atoms (hydrogen, helium, etc.) have different numbers of atoms ‘orbiting’ them in different shells. The electrons cannot be just any random distance from the electron but MUST orbit in prescribed distances from the nucleus (center) of the atom. If you add energy to an atom you will excite the electron. If you provide enough power for the atom to jump to the next level it will…anything less and it will stay put. However, the atom doesn’t want to stay at the higher energy level and prefers to drop back to is ground state. When the atom drops back down a level (or two or three or whatever) it emits a photon of light. As I mentioned each type of atom is unique in what levels atoms can orbit at. So, when something emits light of a particular wavelength you can determine what it was that made that light.
3. How is a photon destroyed?
Photons are being destroyed all the time. Look at your desk…it’s happening right there. White light hits your desk and it absorbs all wavelengths except a few which it reflects. This reflected light determines what color you perceive your desk to be. The absorbed photon gives its energy to the desk causing your desk to heat up. Of course, there isn’t enough light in your room to significantly heat the desk but it is happening just the same.
As for the electron its anti-particle is a positron (not a photon).
In my answer to question #2 I keep using the word ‘atom’ when I meant ‘electron’ and pooched a few other things as well. It should read like this:
Just above I mentioned that a photon could tell someone what it came from by measuring its energy/wavelength (remember light can be considered both a wave and a particle simultaneously). Atoms have electrons orbiting them. Different types of atoms (hydrogen, helium, etc.) have different numbers of electrons ‘orbiting’ them in different shells. The electrons cannot be just any random distance from the nucleus (center) but MUST orbit in prescribed distances from the nucleus of the atom. If you add energy to an atom you will excite the electron. If you provide enough power for the electron to jump to the next level it will…anything less and it will stay put. However, the electron doesn’t want to stay at the higher energy level and prefers to drop back to is ground state. When the electron drops back down a level (or two or three or whatever) it emits a photon of light. As I mentioned each type of atom is unique in what levels electrons can orbit at. So, when something emits light of a particular wavelength you can determine what it was that made that light.
Thanks, Whack. I caught the typos, and could get what you were saying. I don’t know how the points you made could have been stated more clearly.
I’m still a bit fuzzy about the information thing, though. I’ve usually heard it in conjunction with discussions about shadows travelling faster than light. Like this one, for instance. When the argument about shadows is refuted, the point is inevitably made about information, as in the site I cited: “It is not possible to send information faster than light on a shadow or light spot so FTL communication is not possible in this way.” Could you clear this up a bit for me? Again, thanks for your thoughtful reply.
It should be noted here that we have two different issues being discussed. Photons travel AT the speed of light so do not constitute faster than light communication. If I point a laser at Alpha Centauri I can send a message to them at light speed and my message will arrive there in a little over four years. No problems with that.
Now consider the laser has been on for 4+ years and has arrived at Alpha Centauri (say one of its planets assuming it has any) and they see a little red spot on the ground. If I jiggle my laser back here on earth it will take 4+ years for the spot on the ground in Alpha Centauri to move. However, due to the distances involved, the apparent speed that the red spot moves may be perceived to be WAY above light speed. However, I haven’t sent information faster than light. Even if I arrange with the Centaurians ahead of time that the spot moving east means ‘Yes’ and the spot moving west means ‘No’ they won’t know which is the correct answer for 4+ years from my sending it even thought the spot moved REALLY fast. The spot itself is not information…it is not anything real hence it can ‘break’ the laws of physics in this fashion.
Oops…I forgot to address the FTL communication issue (been a long day).
Anything moving faster than light violates causality. Time stops for anything moving AT the speed of light. From my perspective the photon in my laser sent to Alpha Centauri took 4+ years to arrive. If I rode along with the photon (forgetting for a moment that is not possible) the trip to Alpha Centauri would take zero time. I’d arrive the moment I left.
However, if I went faster than the speed of light then I’d actually be travelling backwards in time (myself or the photon). From my persepctive, going faster than light, I’d arrive at Alpha Centauri before I left. If I could do such a thing I’d make a fortune in tonight’s $300 million lottery. My future self could send the winning numbers back in time to me…but wait, if I won now my future self would already have won and may forget to transmit the winning numbers in his glee! Hmmm – that’s what you call a paradox and one of the good reasons FTL communication isn’t possible.
Shadows can travel faster than light, but the problem is that they don’t really exist; they are just the lack of photons, but not the lack of the same photons at any two moments.
It’s a bit like when one car moves off the front of a of a stationary queue; the car behind moves forward, then the next, and a ‘gap’ between cars can be seen to ‘move’ backwards along the queue, but it’s not actually moving and it’s not the same gap, because gaps aren’t made of anything.
Yes, photons can carry information. Light can be polarized, and the polarization measured when it arrives. Imagine changing the polarization very quickly – that would be equivalent to sending a bunch of 1’s and 0’s. Send enough of them, and presto! You’ve got porn!
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Photons are just little packets of energy (they have no rest mass, although they can have momentum). So, they can be a byproduct of a chemical reaction, or the result of two particles annihilating (the latter would have to produce two photons to satisfy momentum conservation).
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If you have someplace for the energy and momentum to go, then it can be destroyed by most anything. The most common is (as Whack-a-Mole mentioned) the excitation of electrons in atoms. Also, two photons can annihilate and create an electron/anti-electron pair. A photon hitting an electron can scatter off it (meaning they deflect each other like billiard balls), but they don’t cancel each other out or anything.
Yep. Good call. (At first, I was going to argue that you couldn’t get information from a single photon out of direction or position, but then I thought of ways, so I have nothing to say, really.)
There is no single energy that corresponds to one photon. You can have (at least in theory) a photon of any given energy. So the answer to your question is yes, it corresponds to the energy of the right photon.
An energy of a photon can be anything. If a photon with just enough energy to raise the electron to the next level comes along, then that’s what happens. That’s why you get absorption lines - the atoms absorb only photons with that particular energy.
Does that mean that there is a range of discrete values of possible photon energies, or is it a continuously variable scale?
If the photon has more energy than is required for the electron to jump up, Is it possible for the electron to absorb some of it and re-emit the remainder as a photon of a lower energy?
I’m glad you did, Mange. That would have been my next question. I think what has submarined me here is that I’ve discovered that a photon is not a discrete entity, but rather a functional relation. Unless, of course, I’ve misunderstood everything, which is entirely possible.
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*Originally posted by Mangetout *
**Does that mean that there is a range of discrete values of possible photon energies, or is it a continuously variable scale?
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The energy of a photon equals Planck’s constant times the frequency. So the energy is continuously varible.
The simple answer is no, but there are subtleties involved with the uncertainty of energy/time that I don’t think you want to know about. There are things called virtual interactions
The energy of a photon equals Planck’s constant times the frequency. So the energy is continuously variable.
The simple answer is no, but there are subtleties involved with the uncertainty of energy/time that I don’t think you want to know about. Also, there are things called virtual interactions
No, I think you’re quite right; a photon is very unlike the small solid sphere of ‘stuff’ that we might mistakenly imagine it to be. Just thinking about it makes me wonder if anything is real.