# Are particle-antiparticle pairs really one particle looping in time?

1. Jun 3, 2013

### Zmunkz

I am no expert on this stuff, so please forgive any errors.

I've read some Feynman and seen his diagrams showing a particle and antiparticle pair getting spontaneously created, moving apart slightly, then re-combining and annihilating a moment later. The Feynman diagram expresses this as a single closed circle with opposite-facing arrows on each side. I understand this to be equivalent to a single particle moving forward in time, then reversing direction and traveling backwards in time (passing itself as its own antiparticle) back to its originating point, then turning forward in time, again and again.

Can someone explain a little further about this duality, and help me understand if it is really what is happening, or if this is a sortof mathematical artifact? In other words, as far as we know, are particle-antiparticle creations and annihilations really just single particles stuck in small temporal loops, going round and round? Or is it actually two different particles that coincidentally can be described mathematically by reversing time, without any suggestion that time is actually reversing?

Depending on the answers, I will be curious how this cooperates with the MWI of QM, considering the "branching" or worlds occurs only one direction in time, and would seem to breakdown when multiple branches of a single particle are attempting to make their return trip backwards in time.

2. Jun 3, 2013

### Staff: Mentor

Physics cannot answer what "really" happens, that is philosophy. Particles moving backwards in time are a description of a model. If you like this description, use it, otherwise ignore it. It does not change any measurement result if you describe it differently.

This is compatible with MWI.

3. Jun 3, 2013

### Zmunkz

I hear you -- I certainly don't expect to find an answer to the instrumentalism vs realism debate here.

Perhaps you could still elaborate, however. I was more hoping to see if there is a "communal leaning" on this issue. Some models, such as general relativity for instance, are taken by many to likely be real, based on intrinsic simplicity or elegance or whatever you want to call it. Other models like the MWI mentioned above are much more polarized between the instrumental view and the realist view.

In your opinion (based on whatever experience you have), could the time reversal description of the model have any reflection to reality? Or do you think it is so divorced from practical assessment that we should leave it firmly in the hands of philosophy?

I would also be interested to hear a little more about how anything with backwards time fits into the MWI.

Thanks!

4. Jun 3, 2013

### Staff: Mentor

I think you are taking this way too seriously.
Antiparticles are time- and parity-reversed solutions of particles. This does not mean that particles "are going to the future" and antiparticles are "going to the past". The definition of "anti" is arbitrary, we happen to call those particles around us "particles" and the exotic partners "antiparticles" (for obvious reasons), but it could be defined the other way, too. You could say particles are antiparticles, going backwards in time. Would that change any particles? No.

The universe is probably CPT symmetric - the laws of physics are the same (well, nearly) independent of the direction of time. Branching occurs in one way only as the initial state had a low entropy, and systems tend to evolve towards a higher entropy. "Past" is just defined as the direction of lower entropy.

I think collapse interpretations with a literal interpretation of "particles going backwards" could look weird. As seen backwards, those particles, after a measurement, would suddenly get some specific wavefunction in order to fit to some other events afterwards.

5. Jun 3, 2013

### Jolb

I think there are many good reasons why the "particle going back in time" picture, along with the Dirac sea picture, has been relegated to the category of "historical curiosity" rather than useful physical picture. Here is a quote from Zee's Quantum Field Theory in a Nutshell, which develops QFT without reference to these pictures:

(2nd edition p.113)

6. Jun 4, 2013

### tom.stoer

Looking at Feynman diagrams we have two different "types" of objects

1) Real particles and anti-particles i.e. external lines correspond to quantum states and correspond to something we detect in an experimental setup (this is an idealization, but that's how the model works): these real particles and anti-particles move forward in time and have positive energy. Quantum numbers corresponding to charges are reversed for antipartcles (for example the positron has electric charge +e whereas the electron has -e)

2) Virtual particles and anti-particles i.e. internal lines are an artifact of the approximation we use to define Feynman diagrams (perturbation theory); the do not correspond to quantum states (but to propagators), they do not carry physical properties, they are be uniquely defined, and they are not detected in experiments (again this is how the model works): these virtual particles should not be interpreted at all ;-)

Looking at the formalism describing particles and anti-particles the are terms like

$e^{-iEt}$
$e^{+iEt}$

Now it's a matter of taste whether we interpret the second term as

$e^{-i(-E)t}$

with negative energy propagating forward in time or as

$e^{-iE(-t)}$

with positive energy propagating backward in time.

I like the Zee's statement (see the previous post)

Poetic but confusing metaphors

In closing this chapter let me ask you some rhetorical questions. Did I speak of an electron going backward in time? Did I mumble something about a sea of negative energy electrons? This metaphorical language, when used by brilliant minds, the likes of Dirac and Feynman, was evocative and inspirational, but unfortunately confused generations of physics students and physicists. The presentation given here is in the modern spirit, which seeks to avoid these potentially confusing metaphors.

This could be the unique answer to many many posts and questions ...

7. Jun 4, 2013

### Bill_K

And both of these interpretations are antiquated and incorrect, and OMG why do we keep repeating this. We now realize (for at least the last 50-75 years) that φ is not a field but a field operator, and its negative frequency part is an absorption operator for positive energy antiparticles traveling forward in time.

And I like Carlo Rovelli's statement (by way of Naty1, thanks! ) regarding virtual particles:

8. Jun 7, 2013

### mpv_plate

Is absorption operator the same thing as annihilation operator?