A load of questions about photons..

[MadScientist]

Up until recently I believed that photons were charged electrons but as I now understand things..

 

Photons are those packets of energy emitted by atoms when an electron is thrown from one to another, the atom can't contain the new electrons energy so it throws the energy out, is that correct??

 

What happens to the electron though??

Surely it can't just disappear, it must also be thrown out or absorbed mustn't it??

 

And if that's right, another thing I believed was that the photon (charged electron) was passed from one atom to another, like a chain reaction. I assume "real" photons don't travel in this way at all, correct??

All I can find on the web is that photons travel as waves, that's great but means nothing to me.

 

I can understand an atom absorbing a photon, the atom takes that packet of energy and absorbs it in some way transforming it to heat. Does the electron gather more speed causing the atom to vibrate more violently which then spreads to neighbouring atoms, allowing the original atom to slow down slightly. Eventually leaving the original body and entering the air the atoms of which in turn vibrate more violently...

 

A reflected photon would be flung out some way. How does that work, all I can picture is the electron catching it but that kind of atom doesn't like that kind of energy so it throws it back out.

 

What happens when a photon passes through an atom that doesn't absorb it or reflect it?? Does it get absorbed and thrown out or is it more like reflection and instead of being held onto for a full orbit of the electron it's only held onto for half an orbit??

 

And what happens when photons collide?? I assume they absorb each other and form a new photon with more energy.

 

I'm pretty certain that photons of different wavelengths just pass straight through each other, correct??

 

If I'm understanding things, photons don't have to be light we see, this one's confusing me.. Radiation given of by uranium or something like that is just photons of a kind we can't see, correct??

Does that mean that a radio transmitter is emitting photons we can't see but a radio receiver can "see" them??

 

 

Radio telescopes are just gathering photons from distant bodies, right??

And photons travel in straight line.

And to focus on a distant body the telescope has to concentrate on the photons it's emitting, in other words filter out all the photons coming into the telescope from other sources, right??

If so, why are they so wide? Wouldn't it be better to have a really long tube radio telescope instead of a dish and point that at the object

Taken to the extreme a tubular telescope only a photon wide would only collect the photons coming directly from the body it was pointed at, wouldn't it??

 

 

 

And while I'm on the subject of photons, I was reading something more on entangled photons. From what I read they're created by passing light through a certain kind of crystal. That sent my mind racing and I came up with a theoretical invention...

 

Fire the beam of light through the crystal to constantly create entangled photons, for each pair send one off to a distant star and keep the other one trapped between two mirrors. I suppose their surfaces would have to be made from extremely reflective atoms, otherwise over the next 50 to 200 years the photon that stayed at home would dissipate in some way. Perhaps a long fibre optic cable of some kind so you could have a stream of photons constantly travelling in a loop...

 

I was thinking "they" could have been sending photons out and keeping the others to monitor them. An elaborate way of sending a message.

 

And if the receiver trapped these photons some way a direct link to the sender would be achieved.

But it would mean having to overcome the link breaking when interacting with those photons. Could that ever be overcome??

 

 

 

I'm sorry for all the questions but whenever I look at these kind of things on the web, they seem to be assuming I know things about other things, like the "photons travel as waves" as an example I used above.

 

Anyone know of any sites that can take you through the learning process from start to finish?? I'm 36 years old and don't remember Einstein or photons or most of this stuff being mentioned once in school by the teachers, maybe I was off that day.

[Luminol]

Photons are particles of the electromagnetic spectrum that transmit its influence I guess you could say. They aren't electrons...maybe you are thinking about positrons which are antimatter versions of electrons which have a positive charge instead of a negative charge. Anyway...since visible light is only a small part of the electromagnetic spectrum we can't always see the photons... When you think about the electromagnetic spectrum it's the wave explaination that is used to show that different wave frequencies produce different parts of the spectrum.

 

 

Uranium gives off alpha particles, beta particles, and gamma rays.

 

The alpha particles are a helium nucleus that travels at about 5-7% the speed of light. The beta particles are electrons traveling at about 90% the speed of light. Gamma rays are part of the electromagnetic spectrum (very high frequency)...so photons are there.

 

 

Atoms can give off photons when it's electrons jump from a high energy state to a lower one...this is how fluorescence works.

 

I don't know about a website but there is a great book that deals with a lot of these kinds of things...it's called the Elegant Universe by Brian Greene. I haven't read it in a while but it talks about particles, einsteins theories, Quantum mechanics, and then goes into string theory. It's a good book and is just about the easiest to understand...although some of the stuff in there will really get you mind and imagination working...just because that is how the Universe is.

[Severian]

Photons are the force carrying particles of electromagetism, or in more technical terms, they are the gauge bosons of a local U(1) symmetry.

 

Imagine that we had a universe with just electrons in it. All we would have is a universe full of free electrons (and positrons) which do not interact with one another at all, but just buzz about. We could describe this with mathematics easily enough, but we would see that the mathematical formulae had a symmetry. We can transform:

 

[math]\psi \to e^{i \theta} \psi[/math]

 

and nothing would change. This is akin, though not quite the same, to the probabilities in quantum mechanics being proportional to the square of the wavefunction; one can make this phase change without changing the probabilities.

 

We say that the theory so far has a 'global U(1) symmetry'. Let me explain the jargon: U(1) means that the transformation is caused by a Unitary 1x1 matrix, which is just a complex number. 'Global' means that [math]\theta[/math] does not depend on position: the change is made everywhere in the universe at once, by the same amount.

 

Now, this is nice, but not very nice, because we might worry what would happen if I choose to redefine the wavefunction everywhere by an amount [math]\theta_1[/math] while someone in Alpha Centuari, who because of the finite speed of light could not know what I have done, redefines it with [math]\theta_2[/math]. What we really want is for our theory to be 'locally' U(1) symmetric, so that we can change the wavefunction by a phase just around where we are without worrying about anywhere else. In other words we want to transform:

 

[math]\psi \to e^{i\theta(x)} \psi[/math]

 

where [math]\theta[/math] is now a function of position.

 

The mathematics describing the free electrons is not symmetric in this way. To make it locally U(1) symmetric, we find that we need to introduce a new particle which interacts with the electron. By interact, I mean that th electron can absorb it, or emit it.

 

When we work out what properties the particle has to have to give local U(1) symmetry, we find that it has exactly the properties of the photon, and the interaction of the photon with the electron is the electromagnetic force. The photon is a consequence of requiring local U(1) symmetry.

 

The other forces of nature are also the result of making other symmetries of the equations local.

 

(I always find that amazingly beautiful!)

[cyeokpeng]

Photons are not charged particles, they are electrically neutral in net effect. Photons are just electromangnetic waves and it can range from short wavelengths such as gamma and UV, to long wavelengths such as radio waves.

Rememeber that EM waves are sinusoidal-varying electric field orthogonal to a sinusoidal magnetic field orthogonal to the direction of propagation. They are produced when a charge experience acceleration such as an electron vibrating about their equilibrium position due to thermal energy (causing blackbody radaition) or electron jumping from higher energy level to lower energy level (causing your emission line spectra).

 

As to your question when two photons collide, we have to employ the wave-paricle duality in QM. Since we cannot observe individual photons and the photons do not interact with matter, the photons behave as a wave (the wavefunction do not collapse) when they collide heead on or at certain angles, and they superpose in a wave manner. For example, if two photons collide in phase, they will have contructive interference, and if they collide out of phase, they will have destructive interference. Since we do not know where the photons are positioned anywhere in space, according to Heinsenberg uncertainty principle, they is no way we can deduce that in the process of collision, they form a new photon with more energy (counterexample, what about destructive interference case??? they should have less energy) We therefore have to resort to the wave theory of light to explain the effect.

 

Photons of different wavelengths do not just pass through each other. They superpose according to the superposition theorem of waves and produce a completely new wave with different frequency components according to the Fourier theorem.

For example,

two waves, one cos(2*pi*f1*x) another cos(2*pi*f2*x) superpose to give

cos(2*pi*f1*x)+cos(2*pi*f2*x) = 0.5exp(j*2*pi*f1*x)+0.5exp(-j*2*pi*f1*x)

+0.5exp(j*2*pi*f2*x)+0.5exp(j*2*pi*f2*x)

Transform to frequency domain using Fourier transform,

you have 0.5 del(f-f1) + 0.5 del(f+f1) +

0.5 del(f-f2) + 0.5 del(f+f2)

 

In the frequency domain, the resultant superposed wave has two frequency components f1 and f2, and depending on the value of f1 and f2, we may have a fundamental frequency equal to f1 or even smaller.

eg. f1 = 2 * fundamental frequency

f2 = 3 * fundamental frequency

 

Question:

If I'm understanding things, photons don't have to be light we see, this one's confusing me.. Radiation given of by uranium or something like that is just photons of a kind we can't see, correct??

Does that mean that a radio transmitter is emitting photons we can't see but a radio receiver can "see" them??

 

Yes, photons do not have to be the light that we see depending on the wavelength of the photons. Our eye is only sensitive to the visible region of the EM radiation (400nm to aroung 650nm). Don't ask me why this is it. Maybe due to evolution on earth and the nearest star to Earth Sun is emitting EM blackbody radiation most intense in the visible light region. And so most living things on earth evolve to have eyes to be sensitive to the visible light range.

As we know, we can see things by 3 ways, reflection of light from a body, absorption of light and blackbody radiation emission. Actually, most objects on earth are emitting infrared radiation at its peak intensity, and if somehow our eye can detect it, then we can do infrared imaging using our eyes, thereby no such thing as night-blindness anymore.

Radiation from uranium if I'm not wrong is in the gamma wave region which has the highest frequency and shortest wavelength. Our eyes are not sensitive to gamma rays and so such radiation emitted from uranium during radioactive decay is invisible to our eyes.

Yes radio transmitter transmits another type of EM waves with longer wavelength than visible light and lower frequency than visible light. These frequency is are not powerful enough to activate the neuro-sensor in our eyes, and hence radio waves too are invisible to our eyes. However, a radio transmitter is designed to detect radio waves by engineering the antenna to be of the same order of the wavelength of the modulated radio signals, so that the radio receiver can "see" these radio signals. Note that the radio receiver uses electrons in the antenna to detect radio signals, and the antenna material and length is designed carefully to be "sensitive" to the frequency of the modulated radio signal, in a way that the bandgap energy between the conduction band and the valence band is smaller than the energy of the photon, so that the electron can be excited into the conduction band to take part in electrical conduction in the antenna. This conduction will produce I-V characteristics that varies according to your modulated signal received at the receiver side.

[ed84c]

visible light is useful as it is the shortest of the harmeless radiations

[5614]

actually visible light is in the middle of the EM (electromagnetic) spectrum, therefore it is not the shortest wavelength or frequency, infact it is the middle!