Quantum ElectroDynamics:QED by Feynman

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RAPTORATTACK!!!
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Quantum ElectroDynamics:QED by Feynman

Postby RAPTORATTACK!!! » Tue Jul 15, 2008 1:38 am UTC

So, I'm reading his lectures and I'm just about ready, despite the total and overwhelming awesomeness of it all, to bust a nut.

Starting in reverse order:

Antimatter! What? As near as I can tell its when electrons emit a photon before they absorb it and go back in time (which confuses me in itself), during which time they are attracted to electrons, because they are "positively" charged. are they actually? do they exchange photons? why does going backwards in time/having -1 photons make it "positive"? And photons... are their own antiparticle...? This makes no sense to me on so many levels.

On to a very rambling number two: Diffraction from photon absorption. This one should be fairly easy. Ok, So I've figured out why metals are shiny, because the electrons go all over. With the electrons being all over, they have much more chances to diffract an electron back. So... you have Carbon. Similar, aye? a very interconnected lattice all over the place. Yet... Its totally black. not at all shiny. I guess this is because it's rough, so light cant reflect back, and possibly It's black because It's like (table) salt? Salt acts like a diffraction grating for xrays since they are regular bands. But then... why is it white? Are there more electrons because its a salt, as opposed to carbon, which is... something...? Damn. now I've lost it completely. Also, diamond is clear, Graphite is not... confused. HELP! Are the photons in magnetism the same as light photons?

And last I believe... adding and multiplying arrows. Basically, why. Both with regular reflections and in the oh-so-famous these:Image
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Re: Quantum ElectroDynamics:QED by Feynman

Postby recurve boy » Tue Jul 15, 2008 12:37 pm UTC

RAPTORATTACK!!! wrote:Antimatter! What? As near as I can tell its when electrons emit a photon before they absorb it and go back in time (which confuses me in itself), during which time they are attracted to electrons, because they are "positively" charged. are they actually? do they exchange photons? why does going backwards in time/having -1 photons make it "positive"? And photons... are their own antiparticle...? This makes no sense to me on so many levels.

IIRC and I probably don't, it's due to the difference in sign. In this case, since it's all opposite, it can be seen to be a time reversed process. I can't recall if this has much more relevance than in the math. Since most processes, are not time reversible.

Also, diamond is clear, Graphite is not... confused.

Remember that they have different structures. http://en.wikipedia.org/wiki/Image:Diam ... aphite.jpg This also explains why diamond (and anything with similar structures) are is super hard.

Are the photons in magnetism the same as light photons?

Yeah, photons are all the same thing.

And last I believe... adding and multiplying arrows. Basically, why.

You mean vectors?

Man, I was never a great theoretical physicist. But the amount of physics I have forgotten ... oh god.

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Re: Quantum ElectroDynamics:QED by Feynman

Postby RAPTORATTACK!!! » Tue Jul 15, 2008 2:30 pm UTC

recurve boy wrote:
RAPTORATTACK!!! wrote:Antimatter! What? As near as I can tell its when electrons emit a photon before they absorb it and go back in time (which confuses me in itself), during which time they are attracted to electrons, because they are "positively" charged. are they actually? do they exchange photons? why does going backwards in time/having -1 photons make it "positive"? And photons... are their own antiparticle...? This makes no sense to me on so many levels.

IIRC and I probably don't, it's due to the difference in sign. In this case, since it's all opposite, it can be seen to be a time reversed process. I can't recall if this has much more relevance than in the math. Since most processes, are not time reversible.


That's... weird... Protons and antimatter wouldn't react right? Quarks and leptons, plus they repulse? if you hit a neutron with a positron does it turn into a proton and some photons?
Also, diamond is clear, Graphite is not... confused.

Remember that they have different structures. http://en.wikipedia.org/wiki/Image:Diam ... aphite.jpg This also explains why diamond (and anything with similar structures) are is super hard.

how are they so different as to be polar opposites? Carbon nanotubes (made of graphene) have the record for darkest surface. I am really just baffled.
And last I believe... adding and multiplying arrows. Basically, why.

You mean vectors?


EDIT: The reason I asked about photons was I've heard that the photons in atoms
between electrons and the nucleus were virtual photons, and was wondering what that was. Reading about them on wikipedia now.

Note: What comes is a rather long winded and rambling thought process. Basically, I get arrows now, and I was doing some thing stupid before.

He was talking about partial reflections, and shrinking and turning arrows. Sorry, i just dont understand what he's trying to say at all. :?

Basically, you time the distance and time for monochromatic light to the front and back surface of a mirror. The different positions of the arrows let you make a triangle, and the length of the final arrow is the root of the probability of a photon being reflected.

And i read it again and it all makes sense... now I feel silly. Just ignore the question then. I couldnt understand why he had so many parts in what looked like a one step problem, but they are components and i finally understood that. he had arrows for light through the vacuum and I had no clue why, but its okay now. :mrgreen:

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Re: Quantum ElectroDynamics:QED by Feynman

Postby Charlie! » Tue Jul 15, 2008 11:09 pm UTC

Okay, I'll give some things a go
RAPTORATTACK!!! wrote:Antimatter! What? As near as I can tell its when electrons emit a photon before they absorb it and go back in time (which confuses me in itself), during which time they are attracted to electrons, because they are "positively" charged. are they actually? do they exchange photons? why does going backwards in time/having -1 photons make it "positive"? And photons... are their own antiparticle...? This makes no sense to me on so many levels.
Yes, antiparticles of negatively charged things are positively charged.

One interpretation of antimatter (feynman's, kinda) is that it behaves like matter traveling back in time: the repulsion experienced by an electron, when time-reversed, looks like the attraction experienced by an anti-electron.

The photon being it's own antiparticle is pretty much the same as saying it has no antiparticle. And yes, charged particles would exchange photons, because in quantum mechanics photons are the mediators of the electromagnetic force.

On to a very rambling number two: Diffraction from photon absorption. This one should be fairly easy. Ok, So I've figured out why metals are shiny, because the electrons go all over. With the electrons being all over, they have much more chances to diffract an electron back. So... you have Carbon. Similar, aye? a very interconnected lattice all over the place. Yet... Its totally black. not at all shiny. I guess this is because it's rough, so light cant reflect back, and possibly It's black because It's like (table) salt? Salt acts like a diffraction grating for xrays since they are regular bands. But then... why is it white? Are there more electrons because its a salt, as opposed to carbon, which is... something...? Damn. now I've lost it completely. Also, diamond is clear, Graphite is not... confused. HELP! Are the photons in magnetism the same as light photons?
Carbon is not like metal; it forms regular covalent bonds, not metallic bonds. Thus no bunch of free electrons, thus not shiny. ( http://en.wikipedia.org/wiki/Metallic_bond )

A diffraction grating (e.g. salt) has serious problems when the hole size gets below the wavelength size of the light you're shining on it, for one. Also, table salt has enough irregularities to throw light all over. It's actually clear, it just reflects some light off of each grain because it has a high(ish) index of refraction. You can see the clearness in bigger crystals though. ( http://www.geology.neab.net/pictures/rock581.jpg )

Graphite absorbs visible light while diamond passes it because of molecular structure and the differences therein. It's magic!... or maybe quantum mechanics.

Photons in magnetism CAN be the same as the photons picked up by the molecules in our eyes. Virtual particles are weird and cool. ( http://en.wikipedia.org/wiki/Virtual_particle )
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Re: Quantum ElectroDynamics:QED by Feynman

Postby RAPTORATTACK!!! » Wed Jul 16, 2008 1:03 am UTC

Charlie! wrote:One interpretation of antimatter (feynman's, kinda) is that it behaves like matter traveling back in time: the repulsion experienced by an electron, when time-reversed, looks like the attraction experienced by an anti-electron.

The photon being it's own antiparticle is pretty much the same as saying it has no antiparticle. And yes, charged particles would exchange photons, because in quantum mechanics photons are the mediators of the electromagnetic force.


I am now officially satisfied with this. But does this mean we can't have it stable, say, in a block of antimatter? That would be weird.

Charlie! wrote:
On to a very rambling number two: Diffraction from photon absorption. This one should be fairly easy. Ok, So I've figured out why metals are shiny, because the electrons go all over. With the electrons being all over, they have much more chances to diffract an electron back. So... you have Carbon. Similar, aye? a very interconnected lattice all over the place. Yet... Its totally black. not at all shiny. I guess this is because it's rough, so light cant reflect back, and possibly It's black because It's like (table) salt? Salt acts like a diffraction grating for xrays since they are regular bands. But then... why is it white? Are there more electrons because its a salt, as opposed to carbon, which is... something...? Damn. now I've lost it completely. Also, diamond is clear, Graphite is not... confused. HELP! Are the photons in magnetism the same as light photons?
Carbon is not like metal; it forms regular covalent bonds, not metallic bonds. Thus no bunch of free electrons, thus not shiny. ( http://en.wikipedia.org/wiki/Metallic_bond )

A diffraction grating (e.g. salt) has serious problems when the hole size gets below the wavelength size of the light you're shining on it, for one. Also, table salt has enough irregularities to throw light all over. It's actually clear, it just reflects some light off of each grain because it has a high(ish) index of refraction. You can see the clearness in bigger crystals though. ( http://www.geology.neab.net/pictures/rock581.jpg )

Graphite absorbs visible light while diamond passes it because of molecular structure and the differences therein. It's magic!... or maybe quantum mechanics.


ok. Now i wanna find the actual bond lengths for stuff. And for the finer stuff about the optic properties... I'll accept magic unless someone has a better explanation.
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Re: Quantum ElectroDynamics:QED by Feynman

Postby Bravus » Wed Jul 16, 2008 2:28 am UTC

RAPTORATTACK!!! wrote:I am now officially satisfied with this. But does this mean we can't have it stable, say, in a block of antimatter? That would be weird.


Well you COULD have a stable block of anti-matter, though it seems fairly unlikely in our Universe. Our Universe is predominantly matter, so any significant 'chunk' of antimatter would encounter matter and annihilate, IIRC

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Re: Quantum ElectroDynamics:QED by Feynman

Postby mbrigdan » Mon Jul 28, 2008 5:00 pm UTC

Well, you can't just have generic "antimatter", it needs to be anti "something", like anti-hydrogen, or, if you wanted to make it react less often, choose something that rarely, if ever, occurs in nature. Not that I know enough science to name one for you.
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Re: Quantum ElectroDynamics:QED by Feynman

Postby alexh123456789 » Wed Jul 30, 2008 11:07 pm UTC

mbrigdan wrote:Well, you can't just have generic "antimatter", it needs to be anti "something", like anti-hydrogen, or, if you wanted to make it react less often, choose something that rarely, if ever, occurs in nature. Not that I know enough science to name one for you.


Well, antimatter isn't destroyed through a chemical reaction, but an atomic one, so i don't think it'd matter what type of molecule it was in, except for storage purposes.


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