Quantum Physics

Grrrrr · 7 Sep 2008 at 17:21

I've always enjoyed reading and learning about Quantum Physics and General Relativity and with all the talk of the LHC i'm determined to understand as much of it as possible! I've done quite a lot of reading on wikipedia as well as reading Brian Greene's Elegant Universe but sometimes written text is a poor substitute for a good teacher. So i have some questions!

The photon is the force carrier of the electromagnetic force, the W&Z bosons of the Weak Force and Gluons of the strong force. That's easy enough to understand written like that but i can't quite get my head around how the actual process works.

This is how i imagine it to work, so please tell me if i am wrong!

Take for example a 'gun' that can fire 2 electrons parallel to each other, simultaneously. The electrons both have negative charge and repel each other causing them to move apart. So according to the above do the electrons both release a photon in the direction of each other causing them to move apart?

Suppose the gun fires an electron and a proton, they have opposite charges and will move together. In this case will the 2 particles release a photon away from the other particle, thus causing them to move together?

Hopefully this thread will turn into a question and answer session on quantum physics so i'd appreciate it if anyone without an interest in the subject could stay out of the thread. Thanks!

gumbald · 7 Sep 2008 at 17:22

The only quantum stuff I've done was with regards to the computing side of it. Mind-blowing mathematics behind it, not many people can explain it!

Margaret · 7 Sep 2008 at 17:22

When the LHC turns on basically the OcUK headquarters will start operating at full capacity, swallowing the world with the exception of Spie.

Edit: badbob will be present to comment on how poor the sound quality of the world ending was though.

~>Dg<~ · 7 Sep 2008 at 17:25

bit to hardcore for general

more at home with spec me tonights dinner or look i have hair.

Tefal · 7 Sep 2008 at 17:26

I don;t think you can apply Newtonian rules to QM.

Although i have always wondered how positive/negative/magnetics draw things together/repel.

But if they were emitting photons i think we'd be able to detect it wouldn't we?

closeratio · 7 Sep 2008 at 17:29

Margaret said:
When the LHC turns on basically the OcUK headquarters will start operating at full capacity, swallowing the world with the exception of Spie.

Edit: badbob will be present to comment on how poor the sound quality of the world ending was though.

Rofl

I've actually read [art of that book you mentioned by Brian Green btw, but I never finished it either because college work ended up gettin intense or because some of the stuff in there (like the wave-particle duality stuff) I'd already done as part of my physics A-level.

Always been amazed by relativity though. I just find it odd how when you start moving you actually change shape and time, relative to the eye of the beholder of course.

daz · 7 Sep 2008 at 17:32

Grrrrr said:
Take for example a 'gun' that can fire 2 electrons parallel to each other, simultaneously. The electrons both have negative charge and repel each other causing them to move apart. So according to the above do the electrons both release a photon in the direction of each other causing them to move apart?

Suppose the gun fires an electron and a proton, they have opposite charges and will move together. In this case will the 2 particles release a photon away from the other particle, thus causing them to move together?

Hopefully this thread will turn into a question and answer session on quantum physics so i'd appreciate it if anyone without an interest in the subject could stay out of the thread. Thanks!

They exchange "virtual" particles - no "real" photons are created or released etc.

http://en.wikipedia.org/wiki/Force_carrier
http://en.wikipedia.org/wiki/Virtual_particle

Grrrrr · 7 Sep 2008 at 17:54

daz said:
They exchange "virtual" particles - no "real" photons are created or released etc.

http://en.wikipedia.org/wiki/Force_carrier
http://en.wikipedia.org/wiki/Virtual_particle

I've just read the virtual particle article and it explains the exact situation i just posed

From wiki:

In the image above and to the right, the solid lines correspond to real particles (of momentum p1 and so on), while the dotted line corresponds to a virtual particle carrying momentum k. For example, if the solid lines were to correspond to electrons interacting by means of the electromagnetic interaction, the dotted line would correspond to the exchange of a virtual photon. In the case of interacting nucleons, the dotted line would be a virtual pion. In the case of quarks interacting by means of the strong force, the dotted line would be a virtual gluon, and so on.

More Questions soon!

Grrrrr · 7 Sep 2008 at 17:55

Tefal said:
But if they were emitting photons i think we'd be able to detect it wouldn't we?

From the same article as my last post

In practice, a clear distinction can be made: real photons are detected as individual particles in particle detectors, whereas virtual photons are not directly detected; only their average or side-effects may be noticed, in the form of forces or (in modern language) interactions between particles.

Robosapien · 7 Sep 2008 at 18:04

I've found that you have to accept a lot of it at face value for what it is - it's hard to describe it in terms and models that are easy to digest and give you a feeling that you trully understand it in a mechanical sort of way.

Thinking in terms of particles is very limited, you can also think in terms of fields, waves and probabilities.

I don't have the answer, but I'd love for someone in the know to explain it to me aswell. What is the fundamental essence of reality that gives rise to forces? :confused:

Edit: The Feynman diagram have always confused me, since again mechanical intuition prevails - if there is an exchange of momentum, such as in balls colliding with each other, then how does it give rise to an attractive force. It seems to me that they would always cause repulsion.

closeratio · 7 Sep 2008 at 18:13

Robosapien said:
I don't have the answer, but I'd love for someone in the know to explain it to me aswell. What is the fundamental essence of reality that gives rise to forces?

I don't think anyone properly knows the answer to that question. Iirc they know what causes some of the fundamental forces to exist (minus mavity), but they don't know why. Hopefully the LHC should be able to answer some of these questions.

Grrrrr · 7 Sep 2008 at 18:19

Robosapien said:
Edit: The Feynman diagram have always confused me, since again mechanical intuition prevails - if there is an exchange of momentum, such as in balls colliding with each other, then how does it give rise to an attractive force. It seems to me that they would always cause repulsion.

I was pondering this also.

Also, photos clearly exist as particles that we can detect as well as being 'virtual particles'. Can the same be said of bosons and gluons or do they only exist as virtual particles?

Robosapien · 7 Sep 2008 at 18:31

Bosons defintely exist, since it's only a blanket term that includes any particle with an integer spin.

The 'parallel?' group is fermions - any particle with half integer spin.

Bearing in mind that particles can be made up of constituent particles, you can use the description boson/fermion to describe, for example, protons and neutrons, and also quarks which make them up. (Can't recall which group they fall into, just giving an example)

Not sure about the specific bosons involved as messenger particles.

ollie_h · 7 Sep 2008 at 18:56

Your analogy's not too bad, but not quite there. Rather than emitting away from each other in the attractive case, it's more accurate to think of them emitting a negative-virtual-photon towards each other.

It's like this: Imagine two skaters on a pond (these are your massive particles). The skaters want to (nay, have to) interact, and they do this with a bowling ball that skater A is carrying. He chucks it at B and in chucking it is himself pushed away from B, then when B catches it he's pushed away from A. That's the repulsive case which you had fine. The attractive case still involves a bowling ball being chucked, but you can think of it in two ways. Either the ball has negative mass (so you'd follow it when you chucked it) or (the crazier way), is that the ball has a note attached to it which tells skater B to come toward A. Both ways of thinking about it have their uses.

Pre-empting your questions, in this case "mass" actually represents whatever kind of property you want, be it mass, electric charge or colour charge. And the energy it takes to produce and move the virtual particles just comes out of the vacuum. Which you *can* do if you do it for a short enough period of time. But that's probably mentioned in the wiki pages.

Grrrrr · 7 Sep 2008 at 19:10

Very interesting, i appreciate your input

With regards "emitting a negative-virtual-photon towards each other" as you put it, is this a good way of describing what happens, or what actually happens? Am i punching above my weight trying to figure all this out?

KittensCake · 7 Sep 2008 at 19:12

Margaret said:
Edit: badbob will be present to comment on how poor the sound quality of the world ending was though.

Hahahahahaha lmfao

Fusion · 7 Sep 2008 at 19:14

With regards to the skater analogy, when considering attractive forces it may be helpful to think of skater A throwing a boomerang that circles round B, pushing him towards A.

Can't wait to start my High-Energy and Particle Physics module of my degree in October, I find the area fascinating and it will tie in topically with the LHC.

Inquisitor · 7 Sep 2008 at 19:35

closeratio said:
I don't think anyone properly knows the answer to that question. Iirc they know what causes some of the fundamental forces to exist (minus mavity), but they don't know why. Hopefully the LHC should be able to answer some of these questions.

Science can never answer a 'why' question

Mik3 · 7 Sep 2008 at 19:44

If your bored of reading text there's some good physics lectures on YouTube fromthe Berkley university, some cover quantum physics, they are quite basic though.