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Author Topic: What is the difference between an electron and a photon?  (Read 18521 times)

Offline Ethos_

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Re: What is the difference between an electron and a photon?
« Reply #25 on: 20/07/2014 01:38:45 »
It's complicated how Crooke's tube works. Please read
http://en.wikipedia.org/wiki/Crookes_tube#How_a_Crookes_tube_works
Hmm, this article could cause some confusion - it says "When the electrons fall back to their original energy level, they emit light", and you have said "An electron cannot emit a photon", so on the face of it, the article you link to contradicts you.

Perhaps a more detailed or specific explanation would clarify...
After examining this link, I would have to agree with you dlorde. Maybe Pete can clarify this for us?
 

Offline UltimateTheory

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Re: What is the difference between an electron and a photon?
« Reply #26 on: 20/07/2014 03:43:10 »
see the example I gave to describe what the concept of mass-energy equivalence means at
http://home.comcast.net/~peter.m.brown/sr/nuclear_fission.htm

In Eq.5 in equation there should be 931.494 MeV not 952.5 MeV...



And whole equation should give answer:
Uranium-238 -> Thorium-234 + alpha + 4.26992 MeV

Calculation without taking care of the all electrons ionization energies between different isotopes, has large amount of error (from a few eV to few hundred keV difference from actual one).
« Last Edit: 20/07/2014 05:49:26 by evan_au »
 

Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #27 on: 20/07/2014 04:33:58 »
There's a proof reading process that ALL scientific literature MUST go through to catch the errors that are in the papers.

I myself love a text called Classical Mechanics which was originally written by Herbert Goldstein. The first and second versions were written by him. It's a very well know text and found on the shelf of nearly all physicists. The latest version is co-authored by Sakfo and Poole. The proof reading process got screwed up on that book. There was tons of errors. I was catching so many of them that I became a proof reader for the author. When the next printing came out the authors placed a thank you in the acknowledgement section of the text. I ended up catching about 250 errors.

I became friends with a man at MIT, Edwin F. Taylor. At the time I met him he was working on writing a text which was called Scouting Black Holes at the time. Later on Edwin's co-author decided he didn't like that name so I suggested calling it Exploring Black Holes and so it's now named. I found many errors in that text too and a few serious misconceptions. It turned out that one of those misconceptions was one which was in ALL GR texts and probably still is. In fact I'm probably the only person in the world who found an error in Gravitation by Misner, Thorne and Wheeler. The author thought that presence of gravitational redshift was evidence of spacetime curvature, which it isn't. So that error does not appear in that text because of me.

The moral of this story is that my website might contain errors. I never said it was error free. The fact is that it's never been proof read.

An Electron CANNOT emit photons.
« Last Edit: 20/07/2014 12:21:06 by evan_au »
 

Offline UltimateTheory

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Re: What is the difference between an electron and a photon?
« Reply #28 on: 20/07/2014 04:39:14 »
In that equation there should be 931.494061 MeV...
http://en.wikipedia.org/wiki/Atomic_mass_unit

ps3. Uranium-238 decay energy verification:
http://en.wikipedia.org/wiki/Uranium-238
on right side, there is table with value Alpha decay 4.267 MeV...
(the same as mentioned in post #26, and lower by 3% than on your website).
« Last Edit: 20/07/2014 06:08:48 by UltimateTheory »
 

Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #29 on: 20/07/2014 04:59:15 »
Physicists don't memorize all physical constants from all fields.
« Last Edit: 20/07/2014 05:40:51 by evan_au »
 

Offline UltimateTheory

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Re: What is the difference between an electron and a photon?
« Reply #30 on: 20/07/2014 05:04:47 »
I do remember basic constants that I am using in everyday work.
« Last Edit: 20/07/2014 05:39:52 by evan_au »
 

Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #31 on: 20/07/2014 06:37:09 »
Here's an example of errors that creep in conversations like this but wouldn't get past the proof reading process. In Eq. (5) there is a missing 2796af5074a7f27ecccd3cd17e165d53.gif because what appears as 5c2bedad778da166c223d0a0390eec71.gif should have been 793582e3d12a5dbd06dc20a1af435196.gif.
« Last Edit: 20/07/2014 12:16:43 by evan_au »
 

Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #32 on: 20/07/2014 09:08:18 »
In Crookes tube there is not "a single free electron in empty space".
There is beam of electrons, billions of electrons, and they interact with other electrons in beam, colliding each other.
There is electric field, that's not uniform (negative electrode on the left and positive electrode on the bottom).

Any answers in textbooks "can free electron emit photon?" are referring to situation about single particle and nothing else, no electric field, no magnetic field, no other forces, no collisions between multiple particles. Idealized situation in outer space.
For an electron to emit a photon the proper mass of a photon must change during the process and that simply can't happen. The Griffiths text supports this.
« Last Edit: 20/07/2014 12:20:36 by evan_au »
 

Offline Ethos_

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Re: What is the difference between an electron and a photon?
« Reply #33 on: 20/07/2014 13:46:00 »
Thanks for the clarification Pete,..............we are all subject to making errors. It's consistent with the human condition. It's refreshing when we can all learn from each other, makes the world a less lonely place doesn't it.
 

Offline Bill S

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Re: What is the difference between an electron and a photon?
« Reply #34 on: 20/07/2014 14:56:30 »
Hi Ultimate Theory,

As a non-scientist I am here to learn.  Sometimes I struggle with Pete’s posts, but if I ask for an explanation, he provides it.  I’m not really qualified to judge who is right or wrong in the matter of electrons emitting photons, so I have to go with what I understand; what seems to make sense to me. 

So far, Pete’s explanation makes perfect sense to me, whereas yours leaves me shaking my head and thinking “what’s he saying?”

My usual test of whether or not I understand something is if I can explain it to someone else.  At the point we have reached in this thread I could explain why an electron cannot emit a photon, but not the opposite. 

My request would be for less sniping and more explaining.   
 

Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #35 on: 20/07/2014 15:51:35 »
Thanks for the clarification Pete,..............we are all subject to making errors. It's consistent with the human condition. It's refreshing when we can all learn from each other, makes the world a less lonely place doesn't it.
You're quite welcome. While I'm waiting for Griffiths response (if he chooses to provide one) I'll take a shot at what his response might be like.
Quote from: UT
In Crookes tube there is not "a single free electron in empty space".
There is beam of electrons, billions of electrons, and they interact with other electrons in beam, colliding each other.
There is electric field, that's not uniform (negative electrode on the left and positive electrode on the bottom).

Any answers in textbooks "can free electron emit photon?" are referring to situation about single particle and nothing else, no electric field, no magnetic field, no other forces, no collisions between multiple particles. Idealized situation in outer space.
My proof did not depend on how many electrons were in a region of space and the interaction between them because at that level, i.e. in Feynman diagrams, all those forces are accounted for by the exchange of particles, i.e. everything is particles when one is talking about that kind of thing. Griffiths describes what's going on at a vertex of a Feynman diagram. One doesn't  put in electric fields in Feynman diagrams because their existence in those diagrams are implicit due to virtual particles. So Griffiths argument holds in all situations, i.e. not just isolated particles in free space. Had Griffiths been talking only about the special case of isolated particles in free space and not have stated that as an assumption then that would make.

« Last Edit: 20/07/2014 22:08:05 by evan_au »
 

Offline UltimateTheory

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Re: What is the difference between an electron and a photon?
« Reply #36 on: 20/07/2014 18:12:31 »
You're using 952.5 MeV instead of proper 931.494 MeV,
and getting incorrect result of decay energy from Uranium-238 4.3815 MeV instead of proper 4.267 MeV.
I didn't bother about c^2. In decay energy calculation we can ignore it completely, and convert mass of isotope directly to energy by multiplying it by 931.494 MeV, then doing the rest of calcs in energy.
« Last Edit: 20/07/2014 22:04:52 by evan_au »
 

Offline lightarrow

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Re: What is the difference between an electron and a photon?
« Reply #37 on: 20/07/2014 18:26:55 »
I have devices like Crookes tube in the lab...

I suggest you buying or building your own equipment..
http://www.ebay.com/sch/i.html?_nkw=crookes+tube
$70 is cost of Crookes tube + Cockcroft Walton generator producing 40 kV cost $30.

Maybe, but certainly, prior to that, you have to study physics on serious texts or, better, at university, not on popular books.
(The same for jccc).

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« Last Edit: 20/07/2014 22:03:00 by evan_au »
 

Offline UltimateTheory

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Re: What is the difference between an electron and a photon?
« Reply #38 on: 20/07/2014 18:39:51 »
Imagine we have free proton that has rest mass = 938.272 MeV/c^2 (ion of Hydrogen)
and free electron that has rest mass = 0.510999 MeV/c^2
How can free proton intercept free electron?
Electron must emit photon with E=13.6 eV (that's ionization energy needed for Hydrogen)
It'll be attracted to nucleus and form electric neutral Hydrogen.
http://en.wikipedia.org/wiki/Ionization_energy

Spectral lines are described here: http://en.wikipedia.org/wiki/Spectral_line
Read part "Types of line spectra"
Quote "When a photon has about the right amount of energy to allow a change in the energy state of the system (in the case of an atom this is usually an electron changing orbitals), the photon is absorbed."

Wikipedia says that electron is absorbing or emitting photons...

X-Rays article: http://en.wikipedia.org/wiki/X-ray
Quote: "One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted by electrons,"
Quote: "A photoabsorbed photon transfers all its energy to the electron with which it interacts, thus ionizing the atom to which the electron was bound and producing a photoelectron that is likely to ionize more atoms in its path."
(photoelectron is highly accelerated electron which absorbed photon)

Video on YouTube showing different photon-electron, and photon-nucleus reactions:

http://en.wikipedia.org/wiki/Emission_spectrum
Quote: "When the electrons in the atom are excited, for example by being heated, the additional energy pushes the electrons to higher energy orbitals. When the electrons fall back down and leave the excited state, energy is re-emitted in the form of a photon."
« Last Edit: 20/07/2014 22:02:23 by evan_au »
 

Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #39 on: 20/07/2014 22:07:24 »
Quote from: UltimateTheory
Electrons emit photons all the time...
I traced this thread back to the point where it bothered me for the first time and this is it. When you said that electrons emit photons all the time that meant ALL the time, even when they're NOT accelerating. While I know that electrons emit radiate when they're accelerating its clear that accelerating electrons generate the production of photons. Claiming they "emit" photons as if it was a process similar to that of an atom emitting photon when it transitions from a higher state of energy to a lower state of energy and the prior state of energy was reflected in a larger state of proper energy is clearly wrong.

However in any case, your claim that electrons emit photons all the time even when they're not accelerating is clearly wrong. If Griffith responds to my e-mail I suspect that he'll refer to accelerating charges such as bremsstrahlung radiation and cyclotron/synchrotron radiation.

Quote from: UltimateTheory
They just need enough kinetic energy to emit photon.
In this experiment they have few thousands electron volts kinetic energy:
Wrong. If you had an electron traveling at constant speed with 100TeV then it still wouldn't emit photons.

What the mechanism is for electrons to generate photons when they're accelerating is something I don't know. What I do know is that the electron does not absorb energy to increase its proper energy where it later releases it.
 

Offline UltimateTheory

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Re: What is the difference between an electron and a photon?
« Reply #40 on: 20/07/2014 22:30:25 »
You're cutting mine answers to just interesting you part.
The right after I said "electrons emit photons all the time", I wrote:

"They just need enough kinetic energy to emit photon."

Which means that they must be already accelerated.

In Crookes tube case, it's a few thousands volts.

Kinetic energy of electron in electronic circuit with U = 10000 Volts, is at maximum 10 keV energy.
If electron with such huge kinetic energy will hit something, there will be created photon with max 10 keV energy, and electron will be decelerated.
Energy conservation must be obeyed.



When I said "electrons emit photons all the time", I didn't mean that they will emit energy forever and ever. That would obviously violate energy conservation.
If electron emit photon, or other way loses kinetic energy (f.e. resistance of wire), it's decelerated.
It has no longer kinetic energy, and can not give away energy anymore.


« Last Edit: 20/07/2014 23:31:55 by UltimateTheory »
 

Offline UltimateTheory

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Re: What is the difference between an electron and a photon?
« Reply #41 on: 20/07/2014 22:47:53 »
While I know that electrons emit radiate when they're accelerating

Acceleration means also change of momentum. Either one are vectors. Change of direction also means acceleration.

When you have electron going straight path, and then there is positive electrode on bottom (attracting them), beam of electrons must change direction, which is equal to acceleration..

However in any case, your claim that electrons emit photons all the time even when they're not accelerating is clearly wrong.

In Crookes tube they are accelerated by two different mechanisms.. High voltage, and electrodes that're not in straight line.

Wrong. If you had an electron traveling at constant speed with 100TeV then it still wouldn't emit photons.

Again, it's not the case for Crookes tube. They're not going inside it in straight path at constant velocity (velocity is vector, btw, while speed is scalar)... They're bended to bottom.

What the mechanism is for electrons to generate photons when they're accelerating is something I don't know. What I do know is that the electron does not absorb energy to increase its proper energy where it later releases it.

You should always mention "free electron in outer space", not "any electron"...
« Last Edit: 20/07/2014 22:54:40 by UltimateTheory »
 

Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #42 on: 21/07/2014 06:21:07 »
Quote from: UltimateTheory
You're cutting mine answers to just interesting you part.
The right after I said "electrons emit photons all the time", I wrote:

"They just need enough kinetic energy to emit photon."

Which means that they must be already accelerated.
It means that you meant to include it while it was moving at constant velocity after it was accelerated. Just because something was at one time accelerated it doesn't mean that it's accelerated now.

Well an electron moving at constant velocity having 1 Tev of kinetic energy surely has enough energy to emit a photon and therefore according to your claim that "electrons emit photons al the time" they must be emitting photons. And that holds for the situation where the observer is at rest in frame S where the electron is at rest and transforms to the frame where the electron has 1 TeV of kinetic energy. When that observer comes to rest neither he nor observers in his initial frame will observe photons (however observers in an accelerating frame would).
« Last Edit: 21/07/2014 10:11:00 by evan_au »
 

Offline lightarrow

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Re: What is the difference between an electron and a photon?
« Reply #43 on: 21/07/2014 18:10:05 »
If electron emit photon, or other way loses kinetic energy (f.e. resistance of wire), it's decelerated.
It has no longer kinetic energy, and can not give away energy anymore.
An electron goes at constant speed in a circular accelerator.
1. Does it emit synchrotron radiation?
2. Does it lose kinetic energy?
Explain.

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Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #44 on: 24/07/2014 05:13:31 »
Quote from: lightarrow
]An electron goes at constant speed in a circular accelerator.
1. Does it emit synchrotron radiation?
2. Does it lose kinetic energy?
Explain.

1 - That depends on the speed. For ultra-relativistic speeds, i.e. v ~ c then it's called synchrotron radiation. Otherwise it's called cyclotron radiation.

2 - Yes.

Particles accelerating in a circle emit synchrotron radiation when v ~ c. All accelerating charges cause radiation. Its the combination of field + particle that does it much like atom + electron can emit photons.
 

Offline lightarrow

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Re: What is the difference between an electron and a photon?
« Reply #45 on: 24/07/2014 08:33:26 »
Quote from: lightarrow
]An electron goes at constant speed in a circular accelerator.
1. Does it emit synchrotron radiation?
2. Does it lose kinetic energy?
Explain.

1 - That depends on the speed. For ultra-relativistic speeds, i.e. v ~ c then it's called synchrotron radiation. Otherwise it's called cyclotron radiation.
Ok, I didn't know this difference of terminology. Anyway, it emits radiation at every speed.
Quote

2 - Yes.

Particles accelerating in a circle emit synchrotron radiation when v ~ c. All accelerating charges cause radiation. Its the combination of field + particle that does it much like atom + electron can emit photons.
No. I wrote "An electron goes at constant speed, does it lose kinetic energy?" Obviously, it doesn't...

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Offline PmbPhy

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Re: What is the difference between an electron and a photon?
« Reply #46 on: 24/07/2014 10:34:12 »
Quote from: lightarrow
No. I wrote "An electron goes at constant speed, does it lose kinetic energy?" Obviously, it doesn't...
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lightarrow
Right. Thanks for pointing out my slip. You're right of course. Work must be done on the particle because work equals change in kinetic energy.
 

Offline JP

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Re: What is the difference between an electron and a photon?
« Reply #47 on: 29/07/2014 13:13:29 »
This thread is locked since it keeps returning to personal attacks.
 

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