What happens when a particle is absorbed?

[Antuanne]

Particle Being "absorbed"

I've heard this many times before, but what does it mean when a particle is "absorbed"? Does it mean its energy is used up or is it actually absorbed. For instance, I've heard that on the way out of the core of the Sun, a photon is absorbed by a particle multiple times and re-emitted. So, also, what does it mean that a particle is "re-emitted"?

 

[Drakkith]

For photons, absorption means that they interact with and transfer their energy completely to something else, such as a proton, an atom, whatever. They are now GONE. Forever. Re-emission of a photon is...confusing. Your specific example of photons in the Suns core is simply the wrong way of explaining it. Once absorbed each photon is now gone, and it's energy is transferred to whatever particle absorbed it. Then that particle ends up emitting another photon (typically more than one) that is of lower energy than the one that is absorbed.

However re-emission can mean a few other things too. For example atoms and molecules can absorb certain wavelengths of photons and re-emit that exact wavelength right back out. There are also materials such as the "glow in the dark" toys that absorb light and then slowly "re-emit" it back out, thus the glow effect.

 

[kurros]

I've heard this many times before, but what does it mean when a particle is "absorbed"? Does it mean its energy is used up or is it actually absorbed. For instance, I've heard that on the way out of the core of the Sun, a photon is absorbed by a particle multiple times and re-emitted. So, also, what does it mean that a particle is "re-emitted"?

Well usually this would be the kind of language used in atomic physics. Say a photon collides with an atom (really the electrons orbiting said atom). If it is of the appropriate energy, it is destroyed and its kinetic energy used to promote an electron to a higher energy level within the atom. However, the atom is then said to be in an "excited state", and its natural tendency is to relax back to a lower energy state, which is to say that the promoted electron will fall back down to where it started (or some other cascade of electrons dropping to fill the now available lower energy states will occur). During this relaxation the extra energy is "re-emitted" in the form of photons. This is the very basics of atom-photon interactions so there is loads of info about it everywhere: http://en.wikipedia.org/wiki/Excited_state#Atomic_excitation

You can imagine this happening many times throughout the plasma of the sun as a photon tries to travel its way from the core to the surface. Although since it is a plasma there are also lots of interactions between the photons and free charged particles, various forms of scattering and such.

 

[Drakkith]

Kurros, just to be clear, you are using one specific form of absorption/emission, and then talking about a plasma which pretty much doesn't consist of atoms, but of free nuclei and electrons. In this case atomic excitations cannot occur.

 

[kurros]

I haven't checked, but I was assuming that for most of the sun the plasma consisted of only partially ionised atoms, in which case transitions are still occurring, or that at least not all atoms in the plasma are fully ionised all the time.

 

[Drakkith]

I haven't checked, but I was assuming that for most of the sun the plasma consisted of only partially ionised atoms, in which case transitions are still occurring, or that at least not all atoms in the plasma are fully ionised all the time.

I thought the opposite, that the vast majority of the plasma was completely ionized.

 

[kurros]

I thought the opposite, that the vast majority of the plasma was completely ionized.

http://en.wikipedia.org/wiki/Degree_of_ionization#Physics_usage

Ok you probably win, although annoyingly there is no citation for the claim in the wikipedia article, and I am having a hard time getting google to find me something more comprehensive.

 

[mfb]

It depends on the region.

If kT >> binding energy, most atoms are fully ionized, if kT << binding energy, just a small fraction is. Binding energy is ~13 eV for hydrogen and ~50eV for helium (ground state).

In the core, with kT ~ 1keV, nearly all light nuclei are ionized.
At the surface, with kT ~ 1/2 eV, just a small fraction is ionized.

 

[andrien]

when a photon gets absorbed,it means it is destroyed.when it is emitted ,it means a photon has been created(annihilation and creation operator).It does not mean any more in quantum theory of radiation.

 

[robinpike]

when a photon gets absorbed,it means it is destroyed.when it is emitted ,it means a photon has been created(annihilation and creation operator).It does not mean any more in quantum theory of radiation.

So if the photon is destroyed when it is absorbed by an electron, does this mean that the electron - i.e. the electron in general - carries within itself the information necessary to create photons?

 

[Drakkith]

So if the photon is destroyed when it is absorbed by an electron, does this mean that the electron - i.e. the electron in general - carries within itself the information necessary to create photons?

What do you mean by "information"?

 

[robinpike]

What do you mean by "information"?

Well the photon is a thing, and since the assumption is that the photon is destroyed when an electron absorbs a photon - then the question arises that when an electron emits a photon, then where is the information on how to create the photon?

 

[kurros]

Well the photon is a thing, and since the assumption is that the photon is destroyed when an electron absorbs a photon - then the question arises that when an electron emits a photon, then where is the information on how to create the photon?

It is embedded in the laws of the universe that describe how fundamental particles interact with each other.

 

[robinpike]

It is embedded in the laws of the universe that describe how fundamental particles interact with each other.

So are the Laws of the Universe embedded in the electron?

 

[Drakkith]

Let's put it this way, the photon gives energy to the electron, which can then emit another photon using some or all of that energy. It works via the rules of nature, which everything must follow, but I wouldn't necessarily call it "imbedded" in the electron any more than the rules for a lever are imbedded in a see-saw.

 

[robinpike]

Let's put it this way, the photon gives energy to the electron, which can then emit another photon using some or all of that energy. It works via the rules of nature, which everything must follow, but I wouldn't necessarily call it "imbedded" in the electron any more than the rules for a lever are imbedded in a see-saw.

Isn't that a problematic example - for the rules of the lever in a see-saw are a result of the fundamental particles in the see-saw?

Anyway, so is the suggestion that it is energy that has the information of how to create the subsequently emitted photon?

 

[kurros]

Anyway, so is the suggestion that it is energy that has the information of how to create the subsequently emitted photon?

Are you looking for something more specific? Like why does the emitted photon have a particular wavelength? This is just due to conservation of energy in whatever situation is occurring, although this is just one of the said rules that the electron+photon system has to follow. No one knows why these rules exist, it is just how these things have been observed to behave. It is like asking where does the moon get the information about how it is supposed to orbit the earth.

 

[robinpike]

Are you looking for something more specific? Like why does the emitted photon have a particular wavelength? This is just due to conservation of energy in whatever situation is occurring, although this is just one of the said rules that the electron+photon system has to follow. No one knows why these rules exist, it is just how these things have been observed to behave. It is like asking where does the moon get the information about how it is supposed to orbit the earth.

No, I am just asking as a consequence of noting, that if the photon is destroyed when it is absorbed by an electron, then how does the electron produce (i.e. emit) a photon?

I'm not asking how a tally of energy is kept, but the even more basic question of simply:

How is a photon created by an electron?

 

[robinpike]

The assumption that the photon is destroyed when it is absorbed by an electron, raises a far deeper observation than simply the question of how does an electron produce a photon?

That is...

According to chance, it is far more likely that we live in a universe that works using simple fundamental mechanisms, than one that works by complex fundamental mechanisms.

On that basis, for an electron to have the capability to destroy a photon, and for an electron to have the capability to create a photon, is far more complex than say for the electron to have the capability to absorb a photon without destroying the photon, and for the electron to have the capability to emit a photon without having to create it?

 

[Drakkith]

On that basis, for an electron to have the capability to destroy a photon, and for an electron to have the capability to create a photon, is far more complex than say for the electron to have the capability to absorb a photon without destroying the photon, and for the electron to have the capability to emit a photon without having to create it?

I think you are looking too far into it. Also, assuming the electron "has information" is...confusing. The absorption of a photon by an electron or other object is simpy the transfer of energy. Upon emission it is a loss of energy from the emitting object. If you want to call that information, then feel free.

I'm not asking how a tally of energy is kept, but the even more basic question of simply:

How is a photon created by an electron?

It simply does. Classically an EM wave is produced from charged particles through various means, such as acceleration. The quantum description is a bit more complicated but the end result is the same. The electron emits a photon and loses energy. There really isn't a "deeper" understanding than this. (As far as I know)

 

[John Huang]

On that basis, for an electron to have the capability to destroy a photon, and for an electron to have the capability to create a photon, is far more complex than say for the electron to have the capability to absorb a photon without destroying the photon, and for the electron to have the capability to emit a photon without having to create it?

I like your idea. Do you mean a photon stays inside an electron when it is absorbed and gets out when it is emitted? We can then ignore the information of building a photon becasue a photon will exist forever in that situation.

Another wild but similar guess of your idea is that a photon just reduces its frequency when it kicks an electron to a higher orbit and increases its frequency when it is kicked by an electron dropping back to a lower orbit. However, we should be able to see a photon of lower frequency gets out of the electron in an absorbing event and approaches the electron in an emitting event. I don't see any report of such phenomenon yet, so, I think this extension of your idea is too wild. What do you think?

 

[mfb]

Do you mean a photon stays inside an electron when it is absorbed and gets out when it is emitted?

That does not work, unless you simply define virtual electrons in Feynman graphs to be something else than electrons. That does not change physics, however.

Another wild but similar guess of your idea is that a photon just reduces its frequency when it kicks an electron to a higher orbit and increases its frequency when it is kicked by an electron dropping back to a lower orbit.

Those processes are possible, but unlikely with a low photon energy. For high photon energies, the electron is usually kicked out of the atom - if it re-emits a photon, this is called compton scattering.

 

[kurros]

Perhaps the field picture will help you. Electrons and photons are just quantised excitations of the electron and photon quantum fields. Creating and destroying photons is really just energy moving into and out of vibrations in these fields. The fields exist everywhere so the information about what electrons and photons are exists everywhere: just dump a bit of energy into the photon field and a photon pops out!

 

[John Huang]

Those processes are possible, but unlikely with a low photon energy. For high photon energies, the electron is usually kicked out of the atom - if it re-emits a photon, this is called compton scattering.

Thanks. For X ray or gamma ray the electron is usually kicked out of the atom and the X ray photon or gamma ray photon reduces its frequency. Do you know if it is possible that an ultraviolet photon will just kick an electron to escalate but not to depart, and the ultraviolet photon will reduce its frequency?

How about the Sunyaev–Zel'dovich effect? I mean, if free electrons can escalate photons in CMB to higher frequencies, could electrons in the filament of a bulb do the same thing?

 

[Darwin123]

Isn't that a problematic example - for the rules of the lever in a see-saw are a result of the fundamental particles in the see-saw?

Anyway, so is the suggestion that it is energy that has the information of how to create the subsequently emitted photon?

"Information" isn't needed to create a photon. Or at least I don't think it is. I am not sure what you mean by information.

I heard of information being carried by photons, but not information forming a photon. I don't know of any particle that requires information to form or be destroyed. It would help if you could cite any article that says that information is necessary to make a photon.

Energy is needed to create a photon. Energy, linear momentum and angular momentum must be conserved when a photon is created or destroyed. An interaction between particles is needed to create a photon. I never heard of information being needed to create a photon.

Information is a quantity that is measured in a unit called "bits". However, the meaning is of information is a "bit" context sensitive.

 

[John Huang]

Energy is needed to create a photon. Energy, linear momentum and angular momentum must be conserved when a photon is created or destroyed. An interaction between particles is needed to create a photon. I never heard of information being needed to create a photon.

If the linier momentum and the angular momentum are the only concern related to that kind of energy used to create a photon then do you mean the energy is a kinetic energy?

If that is not what you mean, then what else could be extra energies used to create a photon? A gravitational energy or electronic potential energy, or a new kind of energy we don't know yet, or a certain combination of all four energies?

Since last century when phicists have focused on (QM, QED) and (SR, GR) they have not digged into "the interaction between particles" which is needed to create a photon.

I think the "information" is "the step by step procedure of the interaction between particles".

I mean, how two particles work together to control the speed of the newborn photon to be exactly c? What is the detail procedure in producing a photon to let it change its energy between the high energy and the low energy per cycle to maintain an average energy of E=hf while f is the frequency of the photon? What is the high energy amount and what is the low energy amount for a photon with average energy E=hf? Or, even the simple task of how do they decide the polarization of a photon? There are a lot of detail in the interaction between particles.

Physicists working on (QM, QED) do not want to know the reason why a photon behave so strangely. Physicists working on (SR, GR) are focuing on the additional odd character of a photon, the zero mass that a photon should have under the spacetime effect, so that they don't have time to dig into the "interaction between particles" which is needed to create a photon. Only a few physicists care about the detail of the interaction, but I am unable to find their papers.

I do like to know more detail of it. Please help. Thanks.

 

[mfb]

Since last century when phicists have focused on (QM, QED) and (SR, GR) they have not digged into "the interaction between particles" which is needed to create a photon.

Quantum field theory (with QED as special case) is all about interactions of particles. QFT is a wonderful tool to describe those interactions. Where is the problem?

how two particles work together to control the speed of the newborn photon to be exactly c?

There is no need to "control" something.

What is the detail procedure in producing a photon to let it change its energy between the high energy and the low energy per cycle to maintain an average energy of E=hf while f is the frequency of the photon? What is the high energy amount and what is the low energy amount for a photon with average energy E=hf?

Those questions do not make sense, and I think you have a misconception about photons here.

There are a lot of detail in the interaction between particles.

And QFT describes that very well.

 

[Darwin123]

If the linear momentum and the angular momentum are the only concern related to that kind of energy used to create a photon then do you mean the energy is a kinetic energy?

Kinetic energy plus the energy corresponding to the rest masses of the particles involved.
Kinetic energy plus the energy corresponding to the rest masses of the particles involved.
As an example, consider the annihilation of an electron by a positron (i.e., antielectron) in an isolated system. Suppose we consider those events where two photons are created, with no intermediate state. The initial energy, including both kinetic energy and rest mass energy, has to equal the kinetic energy of the two photons. Since photons have zero rest mass, one can only talk about the kinetic energy in the final state.

If that is not what you mean, then what else could be extra energies used to create a photon? A gravitational energy or electronic potential energy, or a new kind of energy we don't know yet, or a certain combination of all four energies?/QUOTE]
Kinetic energy and rest mass energy.
If there are other energies involved, then it is not an isolated system. For instance, the two particles could be swallowed by a black hole. Then, some of the energy could come from the gravitational energy. However, the gravitational energy in this case would really come from the rest mass of the black hole.

Since last century when phicists have focused on (QM, QED) and (SR, GR) they have not digged into "the interaction between particles" which is needed to create a photon.

All these lettered topics involve interactions between particles.

QED stands for "quantum electrodynamics". The electromagnetic interaction between particles is the only thing analyzed in QED. QED includes all sorts of methods for determining how and when photons are generated.

SR uses modified versions of the same force laws between particles as in Newtonian physics. GR has field equations which again show how particles interact. All combinations of quantum mechanics and relativity relate to how photons and other particles are created.

I think the "information" is "the step by step procedure of the interaction between particles".

What is a step by step procedure? What makes you think that "creating" a photon takes more than one step?

I mean, how two particles work together to control the speed of the newborn photon to be exactly c?

So far as is known, two particles can't work together "to control the speed of a newborn photon." The speed of light in a vacuum is "c" according to special relativity.

The word "how" does not imply anything controls anything else. "How" is a description of what happens. QED describes the "how" of generating photons.

What is the detail procedure in producing a photon to let it change its energy between the high energy and the low energy per cycle to maintain an average energy of E=hf while f is the frequency of the photon? What is the high energy amount and what is the low energy amount for a photon with average energy E=hf? Or, even the simple task of how do they decide the polarization of a photon? There are a lot of detail in the interaction between particles.

Physicists working on (QM, QED) do not want to know the reason why a photon behave so strangely. Physicists working on (SR, GR) are focuing on the additional odd character of a photon, the zero mass that a photon should have under the spacetime effect, so that they don't have time to dig into the "interaction between particles" which is needed to create a photon. Only a few physicists care about the detail of the interaction, but I am unable to find their papers.

I do like to know more detail of it. Please help. Thanks.

If you can't find their papers, how do you know that there are even a few physicists that "care about the details of the interaction"?

There are entire textbooks on QED. There are hundreds of articles on QED, QM and EM. The authors present QED and EM as the "how" of making photons. QED and QM describe interactions to the extent that experiments using current technology can probe them.

Maybe you are talking about a different type of "how" than most of us are thinking. Perhaps you can explain what type of "how" and "control" that you are talking about.

 

[John Huang]

Those questions do not make sense, and I think you have a misconception about photons here.

A photon of frequency f has energy of E=hf and the energy of that photon has highest energy at crests and lowest energy at troughs in each of all cycles.

I just like to know two things, the first one is what organ of the photon is in charge of gaining/releasing energy in each cycle and the second one is where does the organ get/release the energy along the way it moves at the speed c? If a photon is created by particles then we may dig into the different stages of the creation to find out some answer of above two questions. I believe a complicated object with a breathing organ like a photon must have a structure. What do you think?

If a photon will exist forever then there may be no newborn photon and we don't have a chance to find baby photons at different stages in their creation.

My next question is also something QM or QED does not care. What is the high energy amount and what is the low energy amount for a photon with average energy E=hf? Do you think the low energy is zero or just an energy between zero and E=hf? I don't know.

I also believe that the polarization of a photon should be handled by an organ of a photon different than the breathing organ. I think, for a photon to hit an object accurately (within a laser ray) the breathing organ of a photon must locate at the center tiny kernel and to respond to a polarizing filter the polarizing organ must extend from the kernel to a diatance comparable with its wavelength. What do you think?

 

[Drakkith]

A photon of frequency f has energy of E=hf and the energy of that photon has highest energy at crests and lowest energy at troughs in each of all cycles.

No it doesn't. A photon always has the same energy no matter where in the phase the EM wave is when it is absorbed.

I just like to know two things, the first one is what organ of the photon is in charge of gaining/releasing energy in each cycle and the second one is where does the organ get/release the energy along the way it moves at the speed c? If a photon is created by particles then we may dig into the different stages of the creation to find out some answer of above two questions. I believe a complicated object with a breathing organ like a photon must have a structure. What do you think?

A photon is simply the result of an EM wave interacting in packets of energy which depend on its frequency. It has no substructure.

If a photon will exist forever then there may be no newborn photon and we don't have a chance to find baby photons at different stages in their creation.

Photons have no stages of creation. An EM wave is created when a charged particle is accelerated and this EM wave then transfers energy to another object in the form of a photon.

My next question is also something QM or QED does not care. What is the high energy amount and what is the low energy amount for a photon with average energy E=hf? Do you think the low energy is zero or just an energy between zero and E=hf? I don't know.

Every photon has a specific amount of energy. There are no min or max unless you have a range of EM wave frequencies. Each frequency wave will have different photons, each with the energy E-hf.

I also believe that the polarization of a photon should be handled by an organ of a photon different than the breathing organ. I think, for a photon to hit an object accurately (within a laser ray) the breathing organ of a photon must locate at the center tiny kernel and to respond to a polarizing filter the polarizing organ must extend from the kernel to a diatance comparable with its wavelength. What do you think?

I have no idea what you are trying to say here, but it isn't correct.

 

[Dead Boss]

A photon of frequency f has energy of E=hf and the energy of that photon has highest energy at crests and lowest energy at troughs in each of all cycles.

Nope. Energy is constant for a given photon. You just wrote down the equation. Both h and f are constants (h is universal constant, f is constant for a given photon) so E must also be constant. Keep in mind that frequency tells you how many cycles are there per second and so it does not change during the cycles.

 

[John Huang]

Since photons have zero rest mass, one can only talk about the kinetic energy in the final state.

QED stands for "quantum electrodynamics". The electromagnetic interaction between particles is the only thing analyzed in QED. QED includes all sorts of methods for determining how and when photons are generated.

SR uses modified versions of the same force laws between particles as in Newtonian physics. GR has field equations which again show how particles interact. All combinations of quantum mechanics and relativity relate to how photons and other particles are created.

Maybe you are talking about a different type of "how" than most of us are thinking. Perhaps you can explain what type of "how" and "control" that you are talking about.

Yes, all QED, SR & GR relate to how photons and other particles are created. But, no detail like which part of a photon is created first and which part is created later. They just talk about under what situation a photon of certain energy will be created.

Let me use your statement "Since photons have zero rest mass, one can only talk about the kinetic energy in the final state." to ask you one of the detail characters of a photon. If all a photon has is kinetic energy then which structure a photon uses it to control the speed to run between the fastest and the lowest speed in each cycle? I like to know that knid of detail. A newborn photon may show that character differently at the beginning of the birth.

 

[John Huang]

Nope. Energy is constant for a given photon. You just wrote down the equation. Both h and f are constants (h is universal constant, f is constant for a given photon) so E must also be constant. Keep in mind that frequency tells you how many cycles are there per second and so it does not change during the cycles.

Thanks for your input. How do you explain the stationary wave in a microwave which heat up certain spots of a chocolate quicker?

 

[Drakkith]

Yes, all QED, SR & GR relate to how photons and other particles are created. But, no detail like which part of a photon is created first and which part is created later. They just talk about under what situation a photon of certain energy will be created.

Again, photons do not have parts.

Let me use your statement "Since photons have zero rest mass, one can only talk about the kinetic energy in the final state." to ask you one of the detail characters of a photon. If all a photon has is kinetic energy then which structure a photon uses it to control the speed to run between the fastest and the lowest speed in each cycle? I like to know that knid of detail. A newborn photon may show that character differently at the beginning of the birth.

Photons do not have kinetic energy, they only carry energy as momentum, and certainly don't vary in speed in this manner. I don't know what cycles you are referring to. The frequency of the EM wave?

Thanks for your input. How do you explain the stationary wave in a microwave which heat up certain spots of a chocolate quicker?

Microwaves do not heat foods evenly because the radiation is sent out from one location in the wall and bounces around the cavity unevenly. It has nothing to do with different photon energies.

 

[mfb]

Yes, all QED, SR & GR relate to how photons and other particles are created.

No, SR and GR do not do that.

But, no detail like which part of a photon is created first and which part is created later.

A photon is an elementary particle, it has no smaller parts (at least according to all measurements done so far).

They just talk about under what situation a photon of certain energy will be created.

A theory which describes all our observations. The ultimate goal of physics (if there would be nothing else but electromagnetism). What else do you want?

If all a photon has is kinetic energy then which structure a photon uses it to control the speed to run between the fastest and the lowest speed in each cycle?

There are no cycles. Just forget that. It is wrong.