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What makes attraction?

by ParticleBane
Tags: attraction, makes
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K^2
#19
Dec6-12, 08:15 AM
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Quote Quote by Bill_K
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.
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.

You can't claim that a particle propagates backwards in time if it has forwards-in-time 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.
andrien
#20
Dec7-12, 07:25 AM
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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.
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?
Bill_K
#21
Dec7-12, 10:21 AM
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Bill,Have not you heard about dirac sea?
andrien, Please reread my post above, #15, where I gave reasons why the Dirac sea is inconsistent with modern theory. Add to them the fact that the Dirac sea does not work with even spin particles (which also have antiparticles!), and that Dirac himself quickly abandoned the idea when the problems with it were pointed out.

(By the way, please don't confuse the Dirac sea in quantum field theory with the Fermi sea in solid state physics, which is perfectly valid and useful.)
Also have not you heard about feynman absorber theory which uses half advanced and half retarded waves?
Again, this was a short-lived toy theory. Note that choice of boundary conditions is irrelevant to discussion of antiparticles. It applies only to internal lines (virtual particles, propagators). These are not solutions of the free particle wave equation, and not part of the Hilbert space of states. A virtual particle can have any 4-momentum, timelike, spacelike or null, future- or past-pointing, and in fact its momentum gets integrated over when the amplitude is calculated. This does not imply that "positrons travel backwards in time," as they are not free particle states, which are the things we observe.
TrickyDicky
#22
Dec7-12, 10:52 AM
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Bill you seem to reject the negative energy notion in the quantum physics threads with the same energy(npi) that you defend it in the relativity threads, like in "Temperature , tensors, and the unruh effect" thread.
DrDu
#23
Dec7-12, 11:04 AM
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Explanation of attraction and repulsion of charges does not require relativistic theory and even less a discussion of the interpretation of states of negative energy.
TrickyDicky
#24
Dec7-12, 11:42 AM
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Quote Quote by DrDu View Post
Explanation of attraction and repulsion of charges does not require relativistic theory and even less a discussion of the interpretation of states of negative energy.
It's been clear from the start of this thread, that such explanation is not known at this point other than a phenomenologic "shut up, get the SM of particles and calculate". So don't be so fast to conclude what eventually might be required to explain it. It might be better to say that currently one doesn't find reasons to think such and such may be required to explain it.
TrickyDicky
#25
Dec7-12, 11:50 AM
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In any case people invariably mentions negative mass or energy when explaining these things:

Quote Quote by DrDu View Post
IYou 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.
Bill_K
#26
Dec7-12, 11:52 AM
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Bill you seem to reject the negative energy notion in the quantum physics threads with the same energy(npi) that you defend it in the relativity threads, like in "Temperature , tensors, and the unruh effect" thread.
Good point. I think the "flux of negative field energy" that's invoked in Hawking radiation in order to explain how the black hole loses mass is also an inward flux of virtual particles. (Since nothing can come out, something must be going in!)
K^2
#27
Dec7-12, 12:23 PM
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Agreed. Off-shell virtual particles carrying negative energy aren't really a problem. Problem with Dirac solutions is that they suggest long range unconfined propagation of negative energy, and that does need a special interpretation.
Vanadium 50
#28
Dec7-12, 07:13 PM
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Quote Quote by ParticleBane View Post
I know someone is going to give me the break down of field theory here but that's not what I'm after.
And what did we do?

I don't want to lock this, so please, if you are going to reply, answer the question asked.
DrDu
#29
Dec7-12, 11:45 PM
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Quote Quote by TrickyDicky View Post
It's been clear from the start of this thread, that such explanation is not known at this point other than a phenomenologic "shut up, get the SM of particles and calculate". So don't be so fast to conclude what eventually might be required to explain it. It might be better to say that currently one doesn't find reasons to think such and such may be required to explain it.
As I tried to lay out, I believe that attraction/ repulsion can be intuitively understood in field theory and certainly do not require a "shut up and calculate" type approach.
My explanation with the "mattress" is based on the introduction of fields in Zee's book "Quantum field theory in a nutshell".
IMHO the point is that attraction/ repulsion is a basic phenomenon in all kinds of field theory, including non-relativistic settings like in solid state theory.
E.g., the phonon assisted attraction of electrons in superconductivity is often depicted by balls rolling on a rubber surface.
So trying to derive it from the SM as you suggest rather hides the underlying physics.
I admit that negative mass is somewhat artificial in my example. In fact I am not sure whether there are examples in particle physics of negative charges coupling to fundamental fields of even spin.
ParticleBane
#30
Dec9-12, 02:56 PM
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Quote Quote by Vanadium 50 View Post
And what did we do?

I don't want to lock this, so please, if you are going to reply, answer the question asked.
I believe I left out a word. I meant to say basic field theory. I wasn't looking for an answer out of a freshman physics book. Something more in depth.
DrDu
#31
Dec9-12, 03:31 PM
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P: 3,593
Btw, we had a very similar thread:
http://www.physicsforums.com/showthread.php?t=401974


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