# What makes attraction?

1. Dec 5, 2012

### ParticleBane

Sorry if this is the wrong place to post this but I wasn't quite sure where to put it.

My question is, what makes thing attract? What actually causes particles to attract to each other? I know someone is going to give me the break down of field theory here but that's not what I'm after. What I want to know is why is there charge and what exactly is it? Also, what causes something to have a charge? Why to things attract to opposite charges and not to like charges? What dictates what is charged and what isn't?

I have many more questions but I feel that this is plenty for the time being.

2. Dec 5, 2012

### K^2

If you don't want a Field-Theoretical answer, you are asking in the wrong place. From perspective of Quantum Mechanics, the reason there is charge is because of the U(1) and SU(2) symmetries of the QED and QCD Lagrangian giving rise to electroweak interactions. As a consequence of that symmetry, there are certain conserved quantities, among them charge. Charge also serves as the source for electromagnetic interaction, which gives you electromagnetic attraction and repulsion via exchange of virtual photons.

Again, if that's not what you're after, you are asking it in the wrong place. Though, I doubt you'll find a more satisfactory answer.

3. Dec 5, 2012

4. Dec 5, 2012

### Naty1

'why things happen' and 'exactly what causes' things is usually beyond the scope of our science. We utilize mathematics to model what we observe, but there are many other
mathematical descriptions that do not seem to apply to our particular universe. Its perhaps a miracle that our man made mathematics applies to our surroundings at all.

5. Dec 5, 2012

### DrChinese

Welcome to PhysicsForums, ParticleBane!

Opposites charges attract, as you say. The explanation of the mechanics behind that is quite complicated, and not something that can be summarized in a short paragraph or two. The explanation of what has charge and what does not is part of what is called the "Standard Model". I think you might find that a good starting point for your reading.

http://en.wikipedia.org/wiki/Standard_Model

6. Dec 5, 2012

### ParticleBane

Thank you. I did enjoy this read although it left me with many more questions that I need to read about.

Thank you for the welcome!

What I really was looking for were some starting points. Starting points are incredibly more helpful than short explanations.

Last edited: Dec 5, 2012
7. Dec 5, 2012

8. Dec 5, 2012

### K^2

Indeed. It does not represent accepted physics of interactions.

9. Dec 5, 2012

### derek101

Think of it as 2 opposites,about a point of equilibrium.

10. Dec 5, 2012

### Naty1

Maybe this explanation espicially Feynman's might offer insights into what we think
happens...or at least accurately explains what we observe.....but it seems to me ere remains some underlying logic we haven't grasped yet.....

http://en.wikipedia.org/wiki/Quantum_electrodynamics

I recall a discussion earlier this year about holes and electrons as charge carriers.....but
just exactly what they were carrying was not discussed.

Saying negative charge is an excess of electrons and positive charge a shortage hardly seems definitive.

11. Dec 5, 2012

### DrDu

It is not generally true that unlike charges attract and like charges repell. Mass is the charge with respect to gavitational interactions and two like masses clearly attract.
The point is that like charges attract if the spin of the particle exchanged is even (like the graviton which has spin 2) and repell if it is odd (like for the photon, which has spin 1).
Nevertheless there is an easy picture: Consider two persons sleeping in the same bed with an old mattrace: The vibrational excitations of the mattrace would be the particles which carry the force in the language of field theory. As each point of the mattrace can move only up or down, this field has only one degree of freedom and therefore has spin 0. Hence two like charges should attract. This is indeed the case: The relevant charge of the two people is their mass. It determines how strongly the mattrace bulges. Obviously the mass of the two people has like sign whence they attract, i.e. they roll to the center of the mattrace. You can also get repulsion: replace one of the two persons by a helium filled balloon placed below the mattrace. The balloon has effectively a negative mass and will bulge the mattrace in the opposite direction. It should be easy to imagine that the ballon and the person repell.

12. Dec 6, 2012

### andrien

it just does not represent accepted physics of interactions(k^2),right!

13. Dec 6, 2012

### K^2

That is Dirac's explanation of electron and positron as well as their charges. If you think of them as particles, such an explanation is quite satisfactory.

However, in field theory formalism, it kind of falls flat. Feynman's interpretation with particle and anti-particle propagating in opposite direction's in time works much better.

Neither of these, however, explains why there is electromagnetic interaction. Merely predicts that the sign of interaction will change depending on whether you are looking at interaction of like or opposite charges.

14. Dec 6, 2012

### derek101

If an electron and positron are considered as 2 opposites about a point of equilibrium,they can head back to the point of equilibrium,but energy must be conserved,so photons can be considered as oscillations about the point of equilibrium,a photon being it's own anti-particle.
Electron/positron annihilation produces photons.

Electron in an atom dropping to lower energy level(heading towards a point of equilibrium) produces photons.

15. Dec 6, 2012

### Bill_K

Are we really going to wonder what antiparticles are all over again? That was not even part of the original post. Oh well...
Both of these explanations are not only flat, but wrong.

Dirac Equation appears to include negative energy solutions. Dirac's Hole Theory to explain this mystery was first formulated in 1930, appearing in print in 1933. Almost immediately in 1934 it was refuted by Heisenberg and others. Only Dirac's reputation preserves it today. The obvious problems: a) the enormous charge density it requires in the vacuum state, and b) the violation of charge symmetry - are electrons the particles and positrons the antiparticles, or is it vice versa? Which is the vacuum state full of??

Feynman's interpretation is slightly better, but introduces its own problems. Sorry - nothing travels backward in time! The best you can say is, "as if". Scattering amplitudes depend continuously on the particle momenta, and one can "twist" particle lines around from future to past, and reuse one calculation to solve another problem.

The modern interpretation is that the "negative energy" solutions found in the Dirac Equation, Klein-Gordon Equation and so on are really just negative frequency solutions, and their presence is a symptom that fields must be replaced by operators. Because in quantum field theory one must specify not only the operators but also the Hilbert space on which they act.

And the correct Hilbert Space contains only positive energy states, with particles of two kinds. The negative frequency part of the field operator does not create a negative energy particle, it destroys a positive energy antiparticle. The two kinds of particle are closely related to each other, but both have positive energy and both travel forward in time.

16. Dec 6, 2012

### andrien

It is just an interpretation,right.positive energy states which will be described as positron are the absence of negative energy states,as one conceive it in hole theory.using backward in time approach is just as good as forward one,it does not violate any principle so far,if causality.

17. Dec 6, 2012

### K^2

And the difference is? Negative frequency means proper time runs in reverse. You just switch which clock runs backwards relative to which. Changes nothing. Weak forces aside, time itself is perfectly symmetric. Backwards or forwards propagation doesn't matter. Now, the boundary conditions, on the other hand, are different. And that means we have a fixed direction for causality. But that isn't influenced in the least by which direction particles are propagating in. A particle propagating in reverse will still have forward scattering.

18. Dec 6, 2012

### Bill_K

No, it is not just an interpretation. There is no such thing as the absence of a state. You cannot have infinity minus one electrons.
The theory is time-reversal invariant, but there is one Hamiltonian operator that propagates the world forward in time, and everything must propagate in the same direction.
You can't claim that a particle propagates backwards in time if it has forwards-in-time boundary conditions.

19. Dec 6, 2012

### K^2

Hamiltonian is perfectly reversible. If Hamiltonian of the universe has no inverse, we are in big trouble. The direction is still all about boundary conditions.

Semantics. We are effectively arguing about phase propagation vs group propagation. I'm saying particle is traveling backwards in time if its phase propagation is reversed. You insist that it's group propagation that determines it. That's just matter of definition of what you call backwards propagation.

20. Dec 7, 2012

### andrien

Bill,Have not you heard about dirac sea?The hole in the sea appears as positron which has positive energy compared to negative energy electron which is in the sea.it also moves forward in time.By the way, it is just an interpretation.Also have not you heard about feynman absorber theory which uses half advanced and half retarded waves?